EXPERIMENTAL  SCIENCE  SERIES  FOR  BEGINNERS. 
I. 

L  I  G-H  T: 


A  SERIES  OF 

SIMPLE,    ENTERTAINING,  AND   INEXPENSIVE   EX- 
PERIMENTS IN  THE  PHENOMENA  OF  LIGHT, 
FOR  THE  USE  OF  STUDENTS 
OF  EVERY  AGE. 


BY 

ALFRED  M.  MAYER  AND  CHARLES  BARNARD. 


NEW   YORK: 
D.    APPLETON    AND    COMPANY, 

549    AND    551    BROADWAY. 
1879. 


COPYRIGHT  BY 

D.  APPLETON  AND  COMPANY, 

187T. 


PEEFATOET  NOTE. 


IT  is  the  design  of  tliis  book  to  furnish  a  number 
of  simple  and  easy  experiments  in  the  phenomena  of 
light,  that  any  one  can  perform  with  materials  that 
may  be  found  in  any  dwelling-house,  or  that  may 
be  bought  for  a  small  sum  in  any  town  or  city.  By 
the  aid  of  this  book  the  reader  becomes  an  experi- 
menter. The  student  of  Nature  may  read  in  books, 
and  soon  forget.  The  experimenter  wTho  questions 
Nature  himself,  who  constructs  his  own  apparatus, 
and  who  performs  his  own  experiments,  learns  past 
forgetting.  He  knows  because  he  has  observed. 

It  is  believed  that  this  book  will  occupy  a  place 
hitherto  unfilled  in  scientific  literature.  It  is  specially 
prepared  for  the  boy  or  girl  student,  and  for  the 
teacher  who  has  no  apparatus  and  who  wishes  his 
pupils  to  become  experimenters,  strict  reasoners,  and 
exact  observers.  Nearly  all  the  experiments  described 
are  new,  and  all  have  been  thoroughly  tested.  The 


4  PREFATORY  NOTE. 

materials  employed  are  of  the  cheapest  and  most 
common  description,  and  all  the  experiments  may  be 
performed  at  an  expense  of  less  than  fifteen  dollars. 
The  apparatus  is,  at  the  same  time,  suitable  for  regu- 
lar daily  use  in  both  the  home  and  school,  and  with 
care  should  last  for  years. 

The  origin  of  this  series  of  books,  and  the  manner 
of  their  production,  may  be  briefly  stated.  For  sev- 
eral years  Professor  Mayer  has  been  studying  how  to 
give  to  every  teacher  and  scholar  the  knowledge  of  the 
art  of  experimenting.  To  accomplish  this  very  desir- 
able object,  he  had  invented  the  simplest  and  cheapest 
apparatus,  and  he  and  his  scientific  friends  had  been 
satisfied  with  their  performance.  It  remained  to 
describe  these  instruments  and  the  ways  of  using 
them.  He  found,  however,  that  his  leisure  from  pro- 
fessional duties  was  not  sufficient  for  this  work,  and, 
not  to  delay  further  the  publication  of  his  labors, 
Professor  Mayer  called  in  Mr.  Charles  Barnard  to 
assist  him  in  preparing  the  books  for  the  press.  The 
construction  and  arrangement  of  the  instruments  were 
explained  to  Mr.  Barnard,  and  the  experiments  were 
made  before  him.  Mr.  Barnard  then  wrote  out  the 
descriptions,  which  were  revised  by  Professor  Mayer. 
The  engravings  have  been  made  under  Professor 
Mayer's  special  direction,  and  care  has  been  taken  to 


PREFATORY  NOTE.  5 

render  them  accurate  representations  of  the  appara- 
tus and  experiments. 

The  nature  of  light  is  not  touched  upon  in  this 
volume.  The  authors  propose  to  explain,  in  another 
book,  the  phenomena  of  interference  and  polarization 
of  light,  and  to  explain  fully  the  structure  of  the  eye 
and  the  nature  of  vision. 

THE  AUTHOKS. 


CONTENTS 


PAGK 

PREFATORY  NOTE  ....  3 


CHAPTER  I. 
Introduction  . 


CHAPTER  H. 

The  Sources  of  Light    .            .  .            .            .            .12 

The  Heliostat          .            .            .  .            .            .            .16 

First  Experiment  with  Heliostat  .            .            .            .          24 

Experiment  with  Cards  and  a  Lamp  .            .            .            .26 

Experiment  with  Shadows         .  .            .            .30 

Experiment  in  measuring  Light       .  .            .            .            .35 


CHAPTER  III. 

Reflection  of  Light        ......  38 

Experiment  in  Multiple  Reflection  .            .            .            .  .44 

Second  Experiment  in  Multiple  Reflection        .            .            .  47 

Experiment  with  Mirror  on  Pulse   .            .            .            .  .48 

Experiment  with  Glass  Tube     .....  50 

Experiments  in  Dispersed  Reflection           .            .            .  .51 

Experiment  with  Jar  of  Smoke              ....  55 

The  Milk-and-Water  Lamp 58 


CONTENTS. 


CHAPTER  IY. 

Refraction  of  Light 59 

The  Water-Lens 71 

Experiments  in  Projection         .....  76 

The  Fountain  of  Fire        .  .  .  .  .  .80 

The  Water-Lantern     .            .            .                        ,            .  84 

The  Solar  Microscope        ......  90 


CHAPTER  V. 

Decomposition  of  Light          .....  92 

Experiments  with  Solar  Spectrum             .            .            .  .95 

The  Color-Top .            .            .            .            .            .            .  99 

Direct  Recomposition  of  the  Colors  of  the  Spectrum       .  .     103 
Experiments  in  Reflected  Colors          .            .            .            .103 

Experiments  in  Contrasted  Colors            .            .            .  .106 

CONCLUSION     .......  Ill 

LIST  OF  APPARATUS  AND  THEIR  COST      .           .           .  .113 


LIGHT. 

CHAPTER  I. 

INTROD  UGTION. 

ALL  about  us  are  men  busy  with  their  various 
trades  and  professions :  sailing  ships,  digging  in  mines, 
making  all  manner  of  useful  tools  and  machinery, 
planting  seeds  and  reaping  harvests,  and  doing  many 
other  works  and  labors  according  to  certain  fixed  rules 
that  they  found  printed  in  books,  or  that  they  learned 
of  others,  or  that  they  discovered  for  themselves.  Each 
one  has  to  do  with  the  physical  phenomena  around 
him.  The  more  he  knows  about  these  phenomena — 
the  more  he  knows  about  things,  their  relation  to 
each  other,  and  their  action  one  upon  another — the 
better  he  can  work.  ,A  knowledge  of  the  phenomena 
of  Nature  is  the  most  important  knowledge  one  can 
have  who  wishes  to  succeed  in  life.  More  than  this, 


10  LIGHT. 

the  observation  of  facts  in  Nature  gives  readiness  of 
perception,  and  study  and  reading  upon  the  causes  of 
these  facts  stimulate  the  mind  to  healthful  and  pleas- 
urable action. 

The  laws  that  govern  the  physical  phenomena 
about  us  were  not  told  to  us  by  ancient  gods,  or 
divinely-instructed  men.  They  were  discovered  by 
experiment  or  observation.  Men  asked  questions  of 
Nature ;  they  watched  her  phenomena,  till  they  felt 
sure  they  saw  a  reason  for  their  action.  Sometimes 
they  did  not  understand  all  that  happened,  and 
made  strange  guesses  at  the  laws  that  governed  the 
happenings.  Other  men  repeated  the  experiments 
and  got  new  answers ;  and  thus,  in  time,  the  truth 
about  things  became  known.  Many  of  these  facts  in 
Nature,  and  the  laws  that  govern  their  action,  are 
now  known  of  all  men.  Others  are  still  obscure,  or 
dimly  known,  and  are  being  investigated  every  day  in 
the  hope  that  they  may  be  better  understood. 

The  farmer,  the  sailor,  the  mechanic,  and  artisan, 
most  familiar  with  these  facts  and  laws  of  Nature,  is, 
other  things  being  equal,  the  most  likely  to  be  suc- 
cessful in  his  work.  You  hope  to  have  a  share  in  the 
world's  work,  and  you  wish  to  study  Nature  and  her 
phenomena.  You  can  read  about  these  things  in 
books.  A  better  way  is  to  make  experiments — to  ask 


INTRODUCTION.  11 

Nature  yourself — to  examine  the  phenomena  of  light, 
heat,  sound,  electricity,  etc.,  to  study  these  phenom- 
ena, and  find  their  causes  for  yourself. 

To  try  an  experiment  means  to  put  certain  things 
in  certain  relations  with  other  things,  for  the  purpose 
of  finding  out  how- they  affect  each  other.  Experi- 
menting is  thus  a  finding  out. 

It  is  the  design  of  this  book  to  tell  you  something 
about  experiments  in  the  phenomena  of  light — to 
show  how  these  experiments  illustrate  the  action  of 
light,  and  to  explain  briefly  some  of  the  elementary 
laws  that  govern  this  action.  All  of  these  experi- 
ments may  be  performed  with  the  cheapest  and  most 
common  materials  that  can  be  found.  They  are  all 
easy  and  simple,  and  they  are,  at  the  same  time,  in- 
teresting and  entertaining.  Some  of  the  things  here 
described  are  capable  of  affording  amusement  for  a 
large  number  of  people,  and  many  of  the  exhibitions 
and  displays  that  may  be  made  with  them  are  won- 
derfully attractive  and  beautiful. 


12  LIGHT. 


CHAPTEE  II. 

THE  SOURCES   OF  LIGHT. 

WHEN  the  sun  rises  in  the  morning,  the  darkness 
of  the  night  seems  to  fade  away,  and,  wherever  we 
look,  without  or  within,  all  the  air  and  space  about  us 
appears  to  be  full  of  light.  "When  evening  comes 
again,  the  daylight  disappears,  and  the  moon  and  the 
stars  give  us  another  light.  In  the  house  we  start 
the  lamps,  and  they  give  us  another  light.  Out-of- 
doors,  in  the  dusky  meadows,  we  see  the  fire-flies  dart- 
ing about,  and  giving  out  pale  sparkles  of  yellow  light 
as  they  fly.  We  look  to  the  north  in  the  night  and 
see  the  aurora,  or  we  watch  the  lightnings  flash  from 
cloud  to  cloud,  and  again  we  see  more  light. 

This  light  from  sun  and  moon,  the  stars,  the  fire, 
the  clouds,  and  sky,  is  well  worth  studying.  It  will 
give  us  a  number  of  the  most  beautiful  and  interest- 
ing experiments,  and,  by  the  aid  of  a  lamp,  or  the 
light  of  the  sun,  we  can  learn  much  that  is  both 
strange  and  curious,  and  perhaps  exhibit  to  our  friends 
a  number  of  charming  pictures,  groups  of  colors, 
magical  reflections,  spectres,  and  shadows.  All  light 
comes  from  bodies  on  the  earth  or  in  the  air.  or  from 


THE  SOURCES  OF  LIGHT.  13 

bodies  outside  of  the  atmosphere ;  and  these  bodies 
we  call  the  sources  of  light.  Light  from  sources  out- 
side of  the  atmosphere  we  call  celestial  light,  and  the 
sources  of  this  light  are  stars,  comets,  and  nebulae. 
The  nebulae  appear  like  flakes  and  clouds  of  light  in 
the  sky,  and  the  comets  appear  only  at  rare  intervals, 
as  wandering  stars  that  shine  for  a  little  while  in  the 
sky  and  then  disappear.  The  stars  are  scattered 
widely  apart  through  the  vast  spaces  of  the  universe, 
and  they  give  out  their  light  both  day  and  night. 
The  brightest  of  these  stars  is  the  sun.  "When  it 
shines  upon  us,  the  other  stars  appear  to  be  lost 
in  the  brighter  light  of  this  greater  star,  and  we 
cannot  see  them.  At  night,  when  the  sun  is  hid, 
these  other  stars  appear.  "We  look  up  into  the  sky 
and  see  thousands  of  them,  fixed  points  of  light,  each 
a  sun,  but  so  far  away  that  they  seem  mere  spots 
and  points  of  light.  Besides  these  stars  are  others, 
called  the  planets,  that  move  round  the  sun.  These 
give  no  light  of  their  own,  and  we  can  only  see 
them  by  the  reflected  light  of  the  great  star  in  the 
centre  of  our  solar  system.  Among  these  stars  are 
the  Moon,  Yenus,  Mars,  Jupiter,  and  many  others. 
"We  might  call  celestial  light  starlight ;  but  the  light 
from  the  great  star,  the  sun,  is  so  much  brighter 
than  the  light  of  the  others,  that  we  call  the  light  it 


14  LIGHT. 

gives  us  sunlight,  and  the  light  from  the  other  suns 
we  call  starlight.  For  convenience,  we  also  call  the 
reflected  light  from  the  planets  starlight,  and  the 
light  from  our  nearest  planet  we  call  moonlight. 

Terrestrial  light  includes  all  the  light  given  out 
by  things  on  the  earth,  or  in  the  air  that  surrounds 
the  earth.  The  most  common  light  we  call  firelight, 
or  the  light  that  comes  from  combustion.  When  we 
light  a  lamp  or  candle,  we  start  a  curious  chemical 
action  that  gives  out  light  and  heat.  The  result  of 
this  action  is  fire,  and  the  light  that  comes  from  the 
flame  is  firelight.  When  a  thunder-storm  rises,  we 
see  the  lightning  leap  from  the  clouds,  and  give  out 
flashes  of  intensely  bright  light.  Sometimes,  at  night, 
the  northern  sky  is  full  of  red  or  yellow  light,  dart- 
ing up  in  dancing  streamers,  or  resting  in  pale  clouds 
in  the  dark  sky.  You  have  seen  the  tiny  sparkles  of 
light  that  spring  from  the  cat's  back  when  you  stroke 
her  fur  in  the  dark,  or  have  seen  the  sparks  that  leap 
from  an  electrical-machine.  All  these — the  aurora, 
the  lightning,  and  the  electric  sparks — are  the  same, 
and  we  call  such  light  electric  light. 

Sometimes,  in  the  night,  we  see  shooting-stars 
flash  across  the  sky.  These  are  not  stars,  but  masses 
of  matter  that,  flying  through  space  about  the  earth, 
strike  our  atmosphere  and  suddenly  blaze  with  light. 


THE  SOUKCES  OF  LIGHT.  15 

The  friction  with  the  air  as  they  dart  through  it  is  so 
great  that  these  masses  glow  with  white  heat,  and 
give  out  brilliant  light.  Two  smooth  white-flint  peb- 
bles, or  two  lumps  of  white  sugar,  if  rubbed  quickly 
together,  will  give  out  light,  and  this  light  we  call 
the  light  from  mechanical  action. 

Sailors  upon  the  ocean  sometimes  see,  at  night, 
pale-yellow  gleams  of  light  in  the  water.  A  fire-fly 
or  glow-worm  imprisoned  under  a  glass  will  show,  in 
the  dark,  bright  spots  of  light  on  his  body.  A  piece 
of  salted  fish  or  chip  of  decayed  wood  will  sometimes 
give  a  pale,  cold  light  in  the  night;  and  certain  chemi- 
cals, like  Bologna  phosphorus  and  compounds  of  sul- 
phur, lime,  strontium,  and  barium,  if  placed  in  the 
sunlight  in  glass  vessels  and  then  taken  into  the  dark, 
will  give  out  dull-colored  lights.  All  these  —  the 
drops  of  fire  in  the  sea,  the  glow-worm,  the  bit  of  de- 
cayed wood,  and  these  chemicals — are  sources  of  the 
light  called  phosphorescence. 

These  are  the  sources  of  light — the  stars,  the  fire, 
electricity,  friction,  and  phosphorescent  substances. 
We  can  study  the  light  from  all  of  them,  but  the 
light  from  the  sun  or  a  lamp  will  be  the  most  con- 
venient. The  light  of  the  sun  is  the  brightest  and  the 
cheapest  light  we  can  find,  and  is  the  best  for  our  ex- 
periments. A  good  lamp  is  the  next  best  thing,  and 


16  LIGHT. 

in  experimenting  we  will  use  either  the  sun  or  a 
lamp,  as  happens  to  be  most  easy  and  convenient. 

THE  HELIOSTAT. 

In  looking  out-of-doors  in  the  daytime  we  find 
that  the  sunlight  fills  all  the  air,  and  extends  as 
far  as  we  can  see.  It  shines  in  at  the  window  and 
fills  the  room.  Even  on  a  cloudy  day,  and  in  rooms 
where  the  sunshine  cannot  enter,  the  light  fills  every- 
thing, and  is  all  about  us  on  every  side.  Now,  in 
studying  light  we  do  not  wish  a  great  quantity.  "We 
want  only  a  slender  beam,  and  we  must  bring  it  into 
a  dark  room,  where  we  can  see  it  and  walk  about  it 
and  examine  it  on  every  side,  bend  it,  split  it  up  into 
several  beams,  make  it  pass  through  glass  or  water, 
and  do  anything  else  that  will  illustrate  the  laws  that 
govern  it. 

Choose  a  bright,  sunny  day,  and  go  into  a  room 
having  windows  through  which  the  sun  shines.  Close 
the  shutters,  curtains,  and  blinds,  at  all  the  win- 
dows save  one.  At  this  window  draw  the  curtain 
down  till  it  nearly  closes  the  window,  and  then  cover 
this  open  space  with  a  strip  of  thick  wrapping-paper. 
Cut  a  hole  in  this  paper  about  the  size  of  a  five-cent 
piece,  and  at  once  you  will  have  a  slender  beam  of 
sunlight  entering  the  hole  in  the  paper  and  falling  on 


THE  SOURCES  OF  LIGHT.  17 

the  floor.  Close  the  upper  part  of  the  window  with 
a  thick  shawl  or  blanket,  and,  when  the  room  is  per- 
fectly dark,  our  slender  beam  of  light  will  stand  out 
clear,  sharp,  and  bright. 

As  soon  as  we  begin  to  study  this  beam  of  light, 
we  find  two  little  matters  that  may  give  us  trouble. 
The  sun  does  not  stand  still  in  the  sky,  and  our  beam 
of  light  keeps  moving.  Besides  this,  the  beam  is  not 
level,  and  it  is  not  in  a  convenient  place.  "We  want 
a  horizontal  beam  of  light,  and  some  means  of  keep- 
ing it  in  one  place  all  day.  An  instrument  that  will 
enable  us  to  do  this,  and  that  can  be  adjusted  to  the 
position  of  the  sun  in  the  sky  at  all  seasons  of  the 
year  and  every  hour  of  the  day,  may  be  readily  made, 
and  will  cost  only  a  small  sum  of  money. 

On  the  next  page  are  several  drawings,  giving 
different  views  of  such  an  instrument  and  some  of 
its  separate  parts.  It  is  called  a  heliostat,  and  we 
shall  find  it  of  the  utmost  value  in  our  experiment- 
ing in  light,  heat,  sound,  electricity,  and  other  branches 
of  physical  science. 

The  first  drawing  represents  a  front-view  of  the 
heliostat.  The  second  drawing  gives  an  end-view, 
and  we  can  now  make  one  by  simply  following  these 
few  directions  :  The  part  marked  A  in  the  two  draw- 
ings is  a  piece  of  pine  board,  23  inches  (58.4  centi- 


2      3     4.     5     6 

SCALE  Ys 


FIG.  1. 


THE  SOURCES  OF  LIGHT.  19 

metres)  wide  and  two  or  more  feet  long,  or  as  long  as 
the  window  where  it  is  to  be  used  is  wide.  Any  boy 
who  can  use  plane  and  saw  can  make  this  piece  of 
work  out  of  common  inch-board,  and,  if  you  have  no 
pieces  so  wide  as  that,  it  can  be  made  of  two  or  more 
pieces  fastened  together  with  cleats ;  but,  in  this  case, 
all  the  cracks  must  be  close  and  tight.  In  the  middle 
of  this  board,  cut  a  round  hole  5  inches  (12.7  centi- 
metres) in  diameter,  with  its  centre  8  inches  from 
the  bottom  of  the  board.  In  the  first  drawing  this 
hole  can  be  seen  at  B,  and  in  the  second  drawing 
it  is  shown  by  dotted  lines  at  B.  On  one  side  of 
the  board  screw  two  iron  brackets,  using  brackets 
measuring  14  inches  (35.5  centimetres)  by  12  inches 
(30.5  centimetres).  These  brackets  are  placed  one 
on  each  side  of  the  hole  in  the  board,  and  are 
placed  14  inches  (35.5  centimetres)  apart,  and  with 
the  short  arm  of  the  bracket  against  the  board.  In 
the  first  drawing  the  two  brackets  are  shown,  and  in 
the  second  drawing  one  is  shown  in  profile,  and  they 
are  marked  C  in  both  drawings.  On  the  end  of  the 
brackets  is  placed  a  flat  piece  of  board,  6J  inches 
(16.5  centimetres)  wide  and  14  inches  (35.5  centi- 
metres) long,  or  long  enough  to  reach  from  one 
bracket  to  the  other.  This  board  may  be  screwed 
up  to  the  brackets,  and  thus  make  a  shelf.  Oare 


20  LIGHT. 

must  be  taken  in  fastening  this  shelf  to  the  brackets 
to  place  it  so  that  the  outside  edge  of  the  shelf  will 
be  16  inches  (40.6  centimetres)  from  the  large  board. 
On  the  outside  edge  of  this  shelf  another  board,  7 
inches  (1T.8  centimetres)  wide,  is  placed  upright,  and 
secured  with  screws  and  small  strips  of  wood  at  the 
ends,  as  in  the  drawing.  This  shelf,  with  the  wooden 
back,  is  marked  D  in  the  drawings. 

These  things  make  the  fixed  parts  of  the  heliostat, 
and  we  have  next  to  make  the  movable  parts,  or  the 
machinery  whereby  it  can  be  adjusted  to  the  move- 
ment of  the  sun  in  the  heavens.  First,  get  out  a  flat 
piece  of  board  10^  inches  (26.7  centimetres)  long,  6J 
inches  (16  centimetres)  wide,  and  J  inch  (12  milli- 
metres) thick.  Then  make  a  flat,  half-round  piece, 
shaped  like  the  figure  marked  G.  This  piece  must 
be  J  inch  (T  millimetres)  thick,  5£  inches  (14  centi- 
metres) along  the  straight  side,  and  with  the  circular 
part  with  a  radius  of  3  inches  (7.6  centimetres).  A 
hole,  £  inch  (12  millimetres)  in  diameter,  is  made  in 
this,  as  represented  in  the  drawing,  and  then  the  half- 
round  piece  must  be  screwed  to  the  flat  piece  of  wood 
we  just  cut  out.  In  the  figure  marked  JVyou  will  see 
these  two  pieces  fastened  together.  Fig.  /  is  the 
most  difficult  piece  of  all.  It  should  be  made  of  ash 
or  some  hard  wood.  One  end  is  square,  and  has  a 


THE  SOURCES  OF  LIGHT.  21 

deep  slot  cut  in  it ;  the  rest  is  round,  and  may  be  1£ 
inch  (32  millimetres)  in  diameter.  The  square  part 
must  be  large  enough  to  slip  over  the  half-circular 
piece,  G,  as  is  shown  at  If.  A  hole,  J  inch  (12  milli- 
metres) in  diameter,  is  cut  in  the  two  ends,  as  marked 
by  dotted  lines  at  J,  and  through  these  holes  an  iron 
bolt  and  nut  are  fitted,  so  as  to  hold  the  circular  piece, 
G,  and  yet  allow  it  to  turn  freely  in  every  direction. 
A  hole,  1J  inch  (32  millimetres)  in  diameter,  is  cut 
through  the  triangular  piece  of  wood  IT,  as  shown 
by  the  dotted  lines,  and  then  this  block  is  securely 
fastened  to  the  back  of  the  large  board,  as  shown  in 
the  second  drawing.  An  opening  of  the  same  di- 
ameter, and  having  the  same  direction,  is  also  cut 
through  the  board,  and  the  movable  piece,  marked 
/,  is  put  through  this  hole,  as  in  the  drawing.  Final- 
ly, we  want  a  wooden  washer,  3£  inches  (8.7  centi- 
metres) wide,  as  represented  at  M.  This  we  slip  over 
the  long  wooden  handle,  as  shown  in  the  second  draw- 
ing, and  this  washer  rests  on  the  block  j^  the  top 
of  which  is  3J  inches  square.  This  makes  all  the 
movable  parts  of  the  heliostat,  and,  when  we  have 
put  in  the  mirrors,  the  instrument  is  finished  and 
ready  for  use.  We  must  have  two  mirrors,  one  6 
inches  (15.2  centimetres)  square  and  one  10  inches 
(25.4  centimetres)  long  and  6  inches  (15.2  centimetres) 


22  LIGHT. 

wide.  These  may  be  made  of  common  looking-glass ; 
but  plate-glass  with  silvered  back  is  far  better,  and 
costs  only  a  little  more. 

Any  carpenter  can  make  this  instrument,  and  the 
cost  will  be  about  as  follows  :  Wood,  50  cents ;  labor, 
$1.75 ;  glass,  $1 ;  iron  nut,  5  cents ;  brackets,  50  cents 
— total,  $3. 80.  When  finished,  the  instrument  should 
have  a  coat  of  shellac-varnish,  and,  when  this  is  done, 
the  mirrors  may  be  put  in  place,  and  fastened  on  with 
very  heavy  bands  of  rubber.  This  will  enable  us  to 
take  the  glasses  off  when  the  instrument  is  not  in  use, 
and,  if  the  elastic  bands  or  rings  are  very  strong,  they 
will  answer  perfectly.  The  long  mirror  is  to  go  on 
the  movable  piece  at  N",  and  the  small  mirror  stands 
on  the  shelf,  facing  the  opening  in  the  board,  at  0. 
This  mirror  stands  at  the  angle  shown  in  the  next 
drawing  (Fig.  2),  and  the  other  mirror  is  adjusted  to 
the  sun  at  its  various  positions  in  the  sky  at  different 
seasons  of  the  year. 

Here  is  a  diagram  showing  the  position  of  the 
handle  of  the  heliostat,  and  the  mirror  for  different 
seasons  and  in  different  parts  of  the  country.  The 
handle  must  be  placed  on  a  line  parallel  with  the 
axis  of  the  earth,  and  the  four  dotted  lines  give  its 
position  when  the  heliostat  is  to  be  used  in  Boston, 
New  York,  Washington,  and  New  Orleans.  This 


THE  SOURCES  OF  LIGHT. 


also  causes  the  block  of  wood  marked  K  to  have  a 
slightly  different  shape,  so  that  the  hole  through  it  will 
be  in  the  middle.  The  dotted  line  marked  "At  Equi- 
nox "  shows  the  path  of  the  light  from  the  sun,  and 


FIG  2. 


the  three  dotted  lines  show  the  paths  of  the  reflected 
light  as  it  passes  from  one  mirror  to  the  other.  The 
position  of  the  movable  mirror  is  also  shown  in  the 
positions  it  has  at  summer  and  winter  solstices. 


24=  LIGHT. 

FIRST  EXPERIMENT  WITH  THE  HELIOSTAT. 

Choose  a  bright  sunny  day,  and  take  the  heliostat 
into  a  room  having  a  window  facing  the  south.  Raise 
the  sash  and  place  the  instrument  in  the  window,  and 
fasten  it  there  so  that  it  will  be  firm  and  steady. 
Before  closing  the  window  down  upon  it,  move  the 
larger  mirror  on  its  axis  till  it  reflects  a  beam  of  light 
into  the  small  mirror.  Then  turn  the  handle  to  the 
right  or  left,  and  a  round,  horizontal  beam  of  light 
will  enter  the  room.  When  this  is  done,  close  all  the 
windows,  so  as  to  make  the  room  as  dark  as  possible. 
To  do  this,  shawls  or  blankets  or  enameled  cloth 
will  be  found  useful  inside  the  curtains  and  shutters. 
Then  get  a  piece  of  cardboard,  about  6  inches  (15.2 
centimetres)  square,  and  lay  a  five-cent  piece  in  the 
centre,  and,  with  a  knife,  cut  a  hole  in  the  card  just 
the  size  of  the  coin.  Then  fasten  this,  with  pins  or 
tacks,  over  the  opening  in  the  heliostat. 

We  have  now  a  slender  beam  of  light  in  a  dark 
room.  Walk  about  and  study  it  from  different  sides. 
See  how  straight  this  slender  bar  of  light  is ;  it  bends  to 
neither  the  left  nor  right,  but  extends  across  the  room 
in  an  absolutely  straight  line.  As  the  sun  moves,  turn 
the  handle  of  the  heliostat  to  keep  the  light  in  place. 

Here  is  a  picture  of  a  dark  room,  in  the  window 


THE  SOURCES  OF  LIGHT. 


25 


of  which  is  the  heliostat.  In  the  centre  of  the  piece 
of  cardboard  is  the  small  hole  where  the  light  enters 
the  room.  A  boy  is  holding  one  end  of  a  long  piece 
of  linen  thread  just  at  the  bottom  of  the  hole  in  the 


FIG.  3. 


card,  and  another  boy  has  drawn  the  thread  out 
straight  and  tight,  so  that  it  just  touches  the  beam  of 
light  throughout  its  length. 

Were  you  to  try  this  experiment,  you  would  see 


26  LIGHT. 

that  the  thread  would  suddenly  be  lighted  up  through- 
out its  whole  length,  and  would  shine  in  the  dark 
room  like  silver.  Then  if  the  boy  allows  the  thread 
tol>ecome  slack  and  loose,  or  if  he  lowers  it  even  a 
very  little,  it  will  disappear  in  the  darkness.  If  he 
raises  and  lowers  it  quickly,  it  will  seem  to  appear 
and  disappear  as  if  by  magic. 

This  is  a  very  pretty  experiment  \  but  we  must 
not  stop  to  look  at  its  merely  curious  effects.  Try  it 
over  several  times,  and  see  if  it  does  not  show  you 
something  about  the  beam  of  sunlight.  Plainly,  if 
the  thread  is  lighted  up  its  whole  length  when  it  is 
straight,  then  the  beam  of  light  must  be  straight  also. 
Here  we  discover  something  about  light;  we  learn 
that  it  has  a  certain  property.  Our  experiment 
shows  that  light  moves  in  straight  lines. 

EXPERIMENT  WITH  CARDS  AND  A  LAMP. 

Here  is  a  picture  representing  three  little  wooden 
blocks  placed  in  a  row  upon  a  flat,  smooth  table,  and 
fastened  to  them  are  three  postal-cards,  so  that  they 
will  stand  upright.  At  the  end  of  the  table  is  a 
small  lamp.  This  is  all  we  need  to  perform  an- 
other experiment,  that  will  show  us  the  the  same 
thing  we  observed  with  the  beam  of  light  from  the 
heliostat.  To  make  these  things,  get  a  piece  of 


THE  SOURCES  OF  LIGHT.  27 

wood  10  inches  (25.4  centimetres)  long,  3  inches 
(76  millimetres)  wide,  and  1J  inch  (37  millimetres) 
thick,  and  saw  it  into  five  pieces,  each  2J  inches 
(64  millimetres)  long.  Next  make  three  slips  of 
pine,  4  inches  (10  centimetres)  long,  3  inches  (76 


FIG.  4. 


millimetres)  wide,  and  £  inch  (4  millimetres)  thick. 
Having  made  these,  get  three  postal-cards,  and  lay 
them  flat  on  a  board,  one  over  the  other.  Just  here 
we  need  a  tool  for  making  small  holes  and  doing 
other  work  in  these  experiments ;  and  we  push,  with 
a  pair  of  pliers,  a  cambric  needle  into  the  end  of  a 
wooden  penholder,  or  other  slender  stick,  putting  the 
eye-end  into  the  wood,  and  thus  making  a  needle- 
pointed  awl.  Measure  off  one-half  inch  from  one  end 


28  LIGHT. 

of  the  top  postal-card,  and  with  the  awl  punch  a 
hole  through  them  all,  just  half-way  from  each  side. 
Lift  the  cards  up,  and  with  a  sharp  penknife  pare 
off  the  rough  edges  of  the  holes,  and  then  run  the 
needle  through  each,  so  as  to  make  the  holes  clean 
and  even. 

Take  one  of  these  cards  and  one  of  the  wooden 
slips,  and  put  the  card  squarely  on  one  of  the  wooden 
blocks  and  place  the  slip  over  it,  and  tack  them  both 
down  to  the  block.  This  will  give  us  the  cards  and 
blocks  as  shown  in  the  picture.  When  each  card  is 
thus  fastened  to  a  block,  we  shall  have  two  blocks 
left.  These  we  can  lay  aside,  as  we  shall  need  them 
in  another  experiment. 

Now  light  the  lamp,  and  place  one  block  on  the 
table,  quite  near  the  lamp.  Look  at  the  lamp  care- 
fully, and  see  that  the  flame  is  just  on  a  level  with  the 
hole  in  the  card.  If  it  is  too  high  or  too  low,  place 
some  books  under  it,  or  put  the  lamp  on  a  pile  of 
books  on  a  chair  near  the  table.  Take  a  chair  and  sit 
at  the  opposite  end  of  the  table,  and  place  another 
card  before  you.  Now  look,  through  the  hole  in  this 
card,  at  the  first  card  before  the  lamp.  If  the  table 
is  level,  you  will  see  a  tiny  star  or  point  of  light 
shining  through  the  holes  in  the  two  cards.  With- 
out moving  the  eye,  draw  the  third  card  into  line 


THE  SOURCES  OF  LIGHT.  29 

between  the  others,  and  in  a  moment  you  will  see 
the  yellow  star  shining  through  all  three  cards. 

Next  take  a  piece  of  thread  and  stretch  it  against 
the  sides  of  the  three  cards,  just  as  they  stand,  and 
immediately  you  see  that  they  are  exactly  in  line. 
The  holes  in  the  cards  we  know  are  at  the  same  dis- 
tance from  the  edges  of  the  cards,  and  our  experiment 
proves  that  the  beam  of  light  that  passed  through  all 
the  holes  must  be  straight,  or  we  could  not  have  seen  it. 
The  cards  are  in  a  straight  line,  and  the  beam  of  light 
must  also  be  straight.  This  experiment,  like  the 
first,  shows  us  that  there  is  a  law  or  rule  governing 
the  movement  of  light,  and  that  law  is,  that  light 
moves  in  straight  lines. 

Move  the  lamp  as  near  to  the  edge  of  the  table  as 
possible,  and  then  bring  one  of  the  cards  close  to  the 
lamp-chimney.  Then  change  your  seat,  and  repeat 
this  experiment  several  times  in  different  directions. 
Each  time  you  will  see  exactly  the  same  thing,  no 
matter  in  what  direction  the  light  moves  from  the 
lamp.  The  lamp  may  be  moved  from  one  side  of 
the  table  to  the  other,  and  in  every  direction  we  shall 
find  the  light  moving  in  exactly  straight  lines  from 
the  source  of  light.  This  is  true  whether  the  source 
be  the  sun,  a  lamp,  or  a  star.  One  can  walk  all  about 
the  lamp  and  see  it  from  every  side,  and  we  can  place 


30  LIGHT. 

our  three  cards  in  any  direction,  north  or  south,  up  or 
down,  east  or  west,  or  in  any  and  every  direction,  and 
every  time  it  will  give  the  same  result. 

Thus  we  have  found  out  the  law  by  which  light 
moves,  viz.,  it  moves  in  straight  lines  in  all  directions 
from  the  source  of  light. 

Knowing  this,  you  can  readily  think  of  a  number 
of  things  in  which  these  laws  are  made  useful.  A 
farmer  planting  an  orchard,  an  astronomer  fixing  the 
positions  of  stars,  a  sailor  steering  his  ship  by  night, 
employs  this  law:  the  first,  to  arrange  his  trees  in 
straight  lines ;  the  second,  to  measure  out  vast  angles 
in  the  sky ;  and  the  third,  to  lay  the  courses  of  his 
ship  in  safety.  Each  employs  these  laws  with  cer- 
tainty and  safety,  because  they  are  fixed  and  never 
change. 

EXPERIMENT  WITH  SHADOWS. 

This  picture  represents  a  sheet  of  white  note-paper, 
standing  upright,  like  a  small  screen,  upon  a  table. 
Near  it  is  a  bit  of  square  paper,  fastened  to  the  end 
of  our  needle-pointed  awl,  and  beside  this  is  a  lamp, 
and  next  to  the  lamp  is  a  postal-card,  having  a  slit 
cut  in  it  near  the  top.  On  the  screen  you  will  no- 
tice that  there  is  a  shadow  of  the  bit  of  paper  held 
on  the  needle.  The  paper  screen  may  be  made  of 


THE  SOURCES  OF  LIGHT.  31 

any  sheet  of  white  paper,  and  it  may  be  held  up- 
right by  placing  some  books  behind  it.  The  bit  of 
paper  on  the  needle  is  just  1  inch  (25  millimetres) 
square;  and  to  hold  the  awl  in  place,  the  handle 


FIG.  5. 


may  be  stuck  in  a  mass  of  wax  on  the  table.  The 
slit  in  the  postal-card  should  be  1  inch  (25  mil- 
limetres) long  and  J  inch  (7  millimetres)  wide,  and 
should  be  horizontal.  The  card  may  rest  against  the 
lamp,  and,  if  it  is  not  high  enough,  put  something 
under  it,  so  that  the  slit  will  be  opposite  the  flame. 
These  things  are  easily  procured,  and,  when  you  have 
them,  light  the  lamp,  place  the  postal-card  before  it, 
and  then  make  the  room  quite  dark,  or,  if  it  is  night, 
put  out  all  the  other  lights.  Set  up  the  needle-awl 
with  the  bit  of  paper  on  the  end  about  12  inches  (30.5 
centimetres)  from  the  lamp,  and  make  it  firm  and 


32  LIGHT. 

steady  with  a  bit  of  wax  softened  in  the  fingers. 
Then  bring  the  screen  in  a  line  with  the  paper  square 
and  the  lamp,  and  about  24  inches  (61  centimetres) 
from  the  lamp.  If  everything  is  right,  there  will  be 
a  square  shadow  of  the  bit  of  paper  on  the  screen. 
Look  carefully  at  everything,  and  have  the  paper  just 
on  a  level  with  the  slit  in  the  postal-card,  and  have 
the  lamp,  paper,  and  screen,  just  in  line,  and  then  the 
square  shadow  will  appear  sharp  and  clear  on  the 
white  screen.  With  a  lead-pencil  trace  an  outline  of 
this  shadow  on  the  screen ;  then  move  the  screen 
just  12  inches  (30.5  centimetres)  farther  from  the 
lamp.  Look  at  the  shadow.  See  how  it  has  increased 
in  size.  With  the  pencil  trace  this  shadow  on  the 
screen,  and  then,  laying  the  screen  on  the  table,  meas- 
ure the  two  shadows,  and  see  how  they  compare  in 
size,  and  see  how  they  both  compare  with  the  size 
of  the  paper  square  that  cast  the  shadows  on  the 
screen. 

Fig.  6  shows  how  light  spreads  out,  and  how 
shadows  expand  as  the  distance  increases.  A  is  the 
lamp,  and  B  is  the  postal-card,  having  a  slit  for  the 
light  to  pass.  C  is  the  paper  screen,  and  D  is  the 
first  shadow  made  on  the  screen  when  it  was  24 
inches  from  the  lamp.  E\&  the  second  shadow  made 
on  the  screen  when  it  was  36  inches  from  the  lamp. 


THE  SOURCES  OF  LIGHT. 


33 


If  you  lay  the  paper  C  on  the  tracing  of  the  small 
shadow  D,  you  will  observe  that  it  only  covers  one- 
fourth  of  the  surface,  and  that  the  shadow  is  four 
times  as  large.  Place  it  on  the  larger  shadow  E,  and 
you  will  see  that  it  covers  only  one-ninth  of  its  sur- 
face. In  the  diagram  the  first  shadow  is  marked  off 
into  quarters,  and  the  second  into  ninths,  by  dotted 
lines.  The  diagram  also  shows  how  the  rays  of  light 
spread  out  wider  and  wider  the  farther  they  travel 
from  the  source  of  light. 

Now,  make  two  squares  of  paper,  one  the  size 
of  D  and  the  other  the  size  of  E.    Then  place  D 


24  inches  (61  centimetres)  from  the  lamp,  and  E 
36  inches  (91.4  centimetres),  and  both  in  a  line.  If 
£7,  D,  and  E>  have  the  positions  shown  in  the  dia- 
gram, it  will  be  found  that  D  and  E  are  both  in 
shadow,  while  the  square  C  is  illuminated.  Eemove 
the  square  (7,  and  D  will  be  lighted,  which  shows 


34  LIGHT. 

that  all  the  light  that  falls  on  D  previously  fell  on 
C.  E  yet  remains  in  darkness.  Next,  remove  D 
and  replace  (7,  and  E  still  remains  in  shade ;  but,  on 
removing  (7,  E\&  fully  illuminated.  This  shows  that 
the  quantity  of  light  that  fell  on  C  spreads  over  four 
times  the  surface  at  the  distance  Z>,  and  nine  times 
the  surface  at  the  distance  E.  Hence  each  one  of  the 
squares  on  D  is  one-fourth  as  bright  as  the  square 
<7,  and  any  one  of  the  squares  on  E  is  one-ninth  as 
bright  as  C. 

Here  we  are  coming  upon  another  fact  about 
light ;  we  find  another  law  governing  its  action. 
At  one  foot  from  the  lamp  the  light  had  a  certain 
power ;  at  two  feet  it  had  only  one-quarter  as  much 
power ;  at  three  feet  it  only  had  one-ninth  as  much 
power  or  intensity.  So,  if  we  approach  the  lamp, 
at  a  certain  distance  the  light  has  a  certain  bright- 
ness ;  at  half  that  distance  it  has  four  times  the 
brightness  ;  at  one-third  the  distance  it  has  nine  times 
as  much  brightness.  The  above  relation,  existing 
between  the  intensity  of  the  light  on  a  surface  at 
different  distances  from  the  source  of  light,  is  often 
stated  as  follows :  The  illumination  of  a  given  sur- 
face varies  in  brightness  inversely  as  the  square  of 
its  distance  from  the  source  of  light. 


THE  SOURCES  OF  LIGHT. 


35 


EXPERIMENT  IN  MEASURING  LIGHT. 

This  picture  represents  a  sheet  of  white  paper, 
standing  upright  upon  a  table.  A  few  inches  from 
this  screen  is  our  needle-pointed  awl,  stuck  upright 
in  the  table.  (If  you  do  not  care  to  do  this,  the  awl 
can  be  stuck  into  a  block  of  wood  or  bit  of  wax.)  A 
lighted  candle  is  placed  on  the  table,  about  22  inches 
(55.8  centimetres)  from  the  screen.  Beyond  this  is  a 
lamp,  placed  upon  a  pile  of  books,  so  as  to  bring  the 


FIG.  7. 


flame  of  the  lamp  on  a  level  with  the  flame  of  the 
candle.  The  lamp  should  stand,  say,  at  44  inches  (112 
centimetres)  from  the  screen ;  and,  if  it  has  a  flat 
wick,  it  must  be  so  placed  that  the  wick  stands  di- 
agonally to  the  screen. 

These  are  all  the  things  we  need  to  make  a  most 


36  LIGHT. 

interesting  experiment  in  measuring  light,  and  we 
have  only  to  make  the  room  dark,  or  put  out  all  the 
other  lights,  if  it  is  evening,  and  we  can  go  on  with 
the  work.  Let  the  candle  burn  a  moment  or  two, 
and  then  bend  the  wick  down,  so  as  to  give  a  large 
flame.  If  you  have  no  lamp,  a  gaslight  will  answer. 
Upon  the  paper  screen  are  two  shadows  of  the  awl 
side  by  side.  Move  the  lamp  to  the  right  or  left  till 
the  two  shadows  just  touch,  and  make  one  broad 
band.  Study  this  double  shadow  carefully.  Per- 
haps one  half  is  darker  than  the  other.  Move  the 
lamp  backward  or  forward,  and  you  will  see  that  its 
shadow  changes — becomes  darker  or  lighter.  Pres- 
ently you  will  find  a  place  for  the  lamp  where  the 
double  shadow  appears  of  a  uniform  depth. 

Now  both  lamp  and  candle  cast  just  as  deep  a 
shadow,  and  yet  one  is  much  farther  from  the  screen 
than  the  other.  Measure  off  the  distance.  Perhaps 
the  candle  is  22  inches  (55.8  centimetres)  from  the 
the  screen,  and  the  lamp  is  44  inches  (112  centi- 
metres). 

In  our  last  experiment  we  found  that  the  illu- 
mination of  a  given  surface  varies  inversely  as  the 
square  of  its  distance  from  the  source  of  light.  The 
square  of  22  is  484,  and  the  square  of  44  is  1,938. 
Now,  if  we  divide  1,938  by  484,  we  get  4,  and  thus 


THE  SOURCES  OF  LIGHT.  37 

we  find  that  our  lamp  is  four  times  as  bright  as  the 
candle.  It  casts  just  the  same  depth  of  shadow  on 
the  screen  as  the  candle,  and  it  is  four  times  as 
bright,  because  the  square  of  the  distance  of  the  can- 
dle will  divide  the  square  of  the  distance  of  the  lamp 
four  times.  If  we  measure  it  another  way  we  find 
the  candle  is  half  the  distance  from  the  lamp  to  the 
screen,  and  gives  only  one-quarter  as  much  light. 

Such  a  measurement  as  this  is  both  easy  and 
simple,  and  by  means  of  such  an  experiment  we  can 
find  out  how  much  light  any  lamp  gives.  In  this 
case,  we  find  one  lamp  gives  just  four  times  as  much 
light  as  the  candle,  or  as  much  light  as  four  candles 
would  give  at  once.  This  is  called  a  photometric 
experiment,  from  two  Greek  words  meaning  light- 
measurement.  You  may  sometimes  hear  people  say 
that  a  certain  gas-lamp  gives  a  sixteen  or  eighteen 
candle  light,  and  our  experiment  shows  us  what  they 
mean  by  this  expression.  They  mean  that  the  lamp 
has  a  photometric  value  of  so  many  candles,  or  gives 
a  light  equal  to  the  light  of  sixteen  or  eighteen  can- 
dles burning  at  the  same  time. 


38  LIGHT. 


CHAPTEE  HI. 

REFLECTION    OF  LIGHT. 

PLACE  the  heliostat  in  position,  and  bring  a  slen- 
der beam  of  light  into  the  darkened  room.  Then 
get  a  small  looking-glass,  or  hand-mirror,  and  a  car- 
penter's steel  square,  or  a  sheet  of  stiff  paper,  having 
perfectly  square  corners.  Hold  the  mirror  in  the 
beam  of  light.  At  once  you  see  there  are  two  beams 
of  sunlight,  one  from  the  heliostat  and  another  from 
the  mirror.  Hold  the  glass  toward  the  heliostat,  and 
you  will  see  this  second  beam  going  back  toward  the 
window. 

This  is  certainly  a  curious  matter.  Our  beam  of 
light  enters  the  room,  strikes  the  mirror,  and  then  we 
appear  to  have  another.  It  is  the  same  beam,  thrown 
back  from  the  glass.  This  turning  back  of  a  beam  of 
light  we  call  the  reflection  of  light. 

Place  a  table  opposite  the  heliostat,  and  place  the 
mirror  upon  it,  against  some  books.  Turn  the  mirror 
to  the  right,  and  the  second  or  reflected  beam  of  light 
moves  round  to  the  right.  Turn  the  glass  still  more, 
and  the  beam  of  light  will  turn  off  at  a  right  angle, 
and  there  will  be  a  spot  of  light  on  the  wall  at  that 


REFLECTION  OF  LIGHT. 


39 


side  of  the  room.  Now  bring  the  carpenter's  square 
or  the  piece  of  square  paper  close  to  the  mirror,  so 
that  the  point  or  corner  will  touch  the  glass  just 
where  the  sunlight  falls  upon  it.  Now  one  edge  of 


FIG.  8. 


the  square  is  brightly  lighted  by  the  sunbeam,  and  if 
the  mirror  is  placed  at  an  angle  of  45  degrees  with 
the  sunbeam,  the  other  edge  of  the  square  is  lighted 
up  by  the  second  beam. 

In  this  diagram,  A  is  the  beam  of  light  from  the 


40  LIGHT. 

heliostat,  and  B  is  the  beam  reflected  from  the  mirror, 
that  is  marked  M.  To  make  this  more  simple,  we 
call  the  first  beam  the  beam  of  incidence,  and  we  say 
that  it  travels  in  the  direction  of  incidence,  as  shown 
by  the  arrow.  The  second  beam,  marked  B,  we  call 
the  beam  of  reflection,  and  the  course  it  takes  we  call 
the  direction  of  reflection.  The  point  marked  0, 
where  the  light  strikes  the  mirror,  is  called  the  point 
of  incidence. 

In  the  diagram  is  a  dotted  line  representing  a 
quarter  of  a  circle  reaching  from  the  beam  of  inci- 
dence to  the  beam  of  reflection.  A  quarter  of  a 
circle,  as  you  know,  is  divided  into  90  degrees. 
Another  dotted  line  extends  from  0  at  the  mirror  to 
X  on  the  quarter-circle,  and  divides  it  into  two  parts. 
Half  of  90  is  45,  and  hence  the  mirror  stands  at  an 
angle  of  45  degrees  with  both  beams  of  light.  Now 
the  line  A  and  the  dotted  line  reaching  from  0  to  X 
make  the  angle  of  incidence,  and  the  angle  between 
B  and  the  line  from  0  to  JTis  the  angle  of  reflection ; 
and  the  curious  part  of  this  matter  is,  that  these  two 
angles  are  always  equal.  Here  they  are  both  angles 
of  45  degrees. 

Move  the  mirror  about  in  any  direction,  and  meas- 
ure the  angles  of  incidence  and  the  angles  of  reflec- 
tion, and  these  angles  will  always  be  exactly  equal. 


REFLECTION  OF  LIGHT. 


41 


If  you  look  at  the  diagram  you  will  see  tliat  the 
mirror  is  at  an  angle  of  45  degrees  with  the  beam  of 
incidence,  and  that  the  beam  of  reflection  is  at  an 
angle  of  90  degrees  with  the  incident  beam.  Hence, 
if  the  mirror  is  tilted  through  a  certain  angle,  the 
reflected  beam  is  tilted  through  twice  this  angle. 
For  instance,  if  the  mirror  is  moved  1  degree  the 


FIG.  9. 


beam  of  reflection  moves  2.  degrees.  Place  the  mir- 
ror at  an  angle  of  22£  with  the  beam  of  incidence, 
and  the  beam  of  reflection  is  at  an  angle  of  45. 
Move  the  mirror  to  an  angle  of  67£,  and  the  beam 
of  reflection  will  move  round  to  an  -angle  of  135 
degrees. 

This  drawing  represents  the  two  postal-cards  fitted 


42  LIGHT. 

on  blocks  of  wood  that  we  used  in  a  former  experi- 
ment, and  the  three  blocks  of  wood  we  cut  out  at 
that  time.  The  five  blocks  are  placed  close  together 
in  a  line,  and  with  the  postal-cards  at  the  ends.  A 
lighted  lamp  is  placed  near  one  of  the  cards,  and  on 
the  middle  block  is  a  small  piece  of  window-glass 
that  has  been  painted  with  black  varnish.  A  single 
coat  of  black  varnish  on  one  side  of  the  glass  is  all 
that  is  required  to  give  us  the  black  mirror  needed  in 
this  experiment.  Place  the  lamp  close  to  the  card  in 
such  a  position  that  the  flame  will  be  just  on  a  level 
with  the  hole  in  the  card.  If  the  lamp  is  not  con- 
venient, the  blocks  and  cards  may  be  placed  upon  a 
table  facing  a  north  window  in  full  daylight. 

When  everything  is  ready,  look  through  the  hole 
in  the  postal-card  marked  B,  down  upon  the  black 
mirror,  and  on  it  you  will  see  a  single  spot  of  light, 
the  reflection  from  the  lamplight  or  the  light  from 
the  window  shining  through  the  hole  marked  A  in 
the  drawing.  Get  the  needle-pointed  awl  and  place 
it  so  that  the  point  will  just  touch  the  spot  of  light 
in  the  black  mirror,  and  then  fasten  the  awl  in  this 
position  with  a  piece  of  wax,  as  represented  in  the 
picture. 

You  will  readily  see  that  this  experiment  is  the 
same  as  the  last.  Again  we  have  a  beam  of  light 


REFLECTION  OF  LIGB^T.  43 

reflected  from  a  mirror.  The  beam  of  incidence 
passes  through  the  postal-card  at  A  and  finds  its  point 
of  incidence  on  the  mirror,  and  the  beam  of  reflection 
extends  from  the  point  of  incidence  to  the  second 
card  at  J?. 

Take  a  sheet  of  stiff  paper  10  inches  (25.4:  centi- 
metres) long,  and  about  4  inches  (10  centimetres) 
wide,  and  hold  it  upright  between  the  two  cards,  with 
the  bottom  resting  on  the  mirror.  .With  a  pencil 
make  a  mark  on  the  edge  of  this  at  the  point  of  in- 
cidence, marked  by  the  awl,  and  at  the  hole  in  the 
card  where  the  beam  of  incidence  enters,  and  marked 
A  in  the  drawing.  Draw  a  line  between  these  two 
points  and  you  have  an  angle  formed  by  this  line  and 
the  base  of  the  paper.  This  angle  marks  the  angle 
of  incidence.  Put  the  paper  on  the  blocks,  with  the 
ruled  line  toward  the  card  -Z?,  and  you  will  find  that 
the  line  fits  here  equally  well.  It  now  extends  from 
the  point  of  incidence  to  B,  and  proves  that  this 
angle  is  the  same  as  the  other,  that  both  sides  are 
alike,  and  that  the  angle  of  incidence  and  the  angle 
of  reflection  are  equal. 

Take  out  the  block  in  the  middle,  and  move  the 
others  nearer  together  till  they  touch.  Repeat  the 
experiment:  make  a  measurement  with  a  piece  of 
paper  as  before,  and  draw  a  line  on  it  from  the  point 


44  ,  LIGHT. 

of  incidence  to  either  of  the  holes  on  the  cards,  and 
then  compare  the  angles  thus  found,  and  in  each  case 
they  will  be  exactly  alike.  Take  out  another  block 
and  try  it  again,  and  you  will  reach  the  same  result. 

These  experiments  show  us  that  there  is  a  fixed 
law  in  this  matter,  and  the  more  we  study  it  the  more 
we  are  convinced  that  it  has  no  exceptions. 

EXPERIMENT  IN  MULTIPLE  REFLECTION. 

Choose  a  south  room  on  a  sunny  day,  and  close 
the  blinds  and  shutters  at  all  the  windows  save  one, 
and  at  this  window  draw  down  the  curtain  until  only 
a  narrow  space  is  left  at  the  bottom.  Close  this  space 
with  a  strip  of  thick  wrapping-paper,  and  then  cover 
the  rest  of  the  window  with  a  blanket  or  shawl,  so 
as  to  make  the  room  perfectly  dark.  Then  cut  a 
round  hole,  the  size  of  a  five-cent  piece,  in  this  paper, 
and  through  this  hole  a  slender  beam  of  sunlight  will 
fall  into  the  darkened  room. 

Bring  a  hand-mirror  into  this  beam  of  light,  and 
the  beam  of  reflection  will  make  a  round  spot  of  sun- 
light on  the  wall  above  the  window.  This  spot  of 
light  is  a  picture  of  the  sun  thrown  by  the  mirror 
upon  the  wall.  Hold  the  mirror  at  an  oblique  angle 
in  the  sunbeam,  and  direct  the  beam  of  reflection 
upon  the  opposite  wall.  Now  there  are  several  re- 


REFLECTION  OF  LIGHT.  45 

flections,  brilliant  spots  of  light.  If  the  spots  of  light 
do  not  stand  out  sharp  and  clear,  turn  the  mirror 
slowly  round  and  you  will  soon  find  a  position  for 
the  glass  that  will  give  six  or  more  reflections. 

How  does  it  happen  that  a  common  looking-glass 
can  thus  split  a  single  sunbeam  into  several  beams  ? 
If  you  touch  a  pencil  to  a  mirror  you  will  notice  that 
while  the  point  of  the  pencil  touches  the  glass  the 
point  of  the  reflected  pencil  sepn  in  the  mirror  does 
not  meet  the  point  of  the  real  pencil,  and  that  there 
is  a  little  space  between  them.  The  reflection  we  see 
in  the  glass  is  from  the  smooth  surface  of  the  quick- 
silver at  the  back  of  the  glass,  and  the  space  between 
the  reflection  and  the  pencil  is  filled  by  the  glass. 

Hold  a  sheet  of  common  window-glass  before  a 
lighted  lamp  or  candle,  and  you  will  see  a  faint  re- 
flection of  the  flame  in  the  glass,  and  at  the  same 
time  you  can  readily  see  through  the  glass.  This 
shows  us  that  the  outside  of  any  piece  of  smooth  glass 
will  reflect  light,  and  our  experiment  is  designed  to 
show  a  still  more  curious  matter. 

Fig.  10  represents  the  single  beam  reaching  the 
point  of  incidence  on  the  outside  of  the  mirror  at  0, 
and  reflected  to  the  wall  at  1.  Part  of  the  light  goes 
through  the  glass  to  B,  and  here  is  another  point  of 
incidence,  and  a  new  beam  of  reflection  is  .thrown 


46 


LIGHT. 


through  the  glass  to  the  wall  at  2.  If  you  look  at  the 
reflections  on  the  wall,  you  will  see  that  the  second 
spot  of  light  is  the  brightest.  This  comes  from  the 
quicksilver,  for,  as  this  is  a  better  reflector  than  the 
glass,  it  sends  out  a  brighter  beam  of  reflection.  When 


FIG.  10. 

this  second  beam  of  reflection  passes  through  the  glass, 
a  part  of  its  light  is  reflected  from  the  under  side  of 
the  surface,  and  is  turned  back  against  the  quicksilver 
again.  Once  more  it  is  reflected,  and  a  new  beam  of 
reflection  makes  number  3.  The  drawing  shows  the 
path  these  beams  of  light  take  in  the  glass,  and  the 
quivering  spots  of  light  on  the  wall  show  how  one 
beam  of  light  may  be  reflected  again  and  again  in  dif- 
ferent directions.  If  the  reflector  was  perfect  and 


REFLECTION  OF  LIGHT.  47 

returned  all  the  light,  these  multiple  reflections  might 
be  repeated  many  times  over ;  but  every  time  light  is 
reflected  from  any  bright  surface,  a  part  of  the  light 
is  lost,  and  thus  each  reflection  grows  fainter  and 
fainter  till  the  light  is  spent.  Look  at  the  multiplied 
reflections  on  the  wall,  and  you  will  see  that  the  first 
reflection  from  the  glass  is  bright,  and  that  the  sec- 
ond, from  the  quicksilver  at  the  back  of  the  glass,  is 
brighter  still ;  and  that  the  others  grow  fainter  and 
fainter  till  all  the  light  is  spent,  and  the  reflections 
disappear. 

SECOND  EXPERIMENT  IN  MULTIPLE  REFLECTION. 

Light  a  lamp  and  place  it  on  a  table,  and  get  the 
two  postal-cards  and  the  blocks  that  we  used  in  the 
experiment  in  reflection.  With  a  sharp  knife  cut  a 
slit  in  one  card,  just  at  the  pin-hole,  about  f  inch  (J9 
millimetres)  long  and  -fa  inc^  (1  millimetre)  wide. 
Then  place  this  card  close  to  the  lamp,  as  in  the  other 
experiment,  and  set  up  the  other  card  about  fifteen 
inches  away  from  it.  Then  lay  a  looking-glass  on  the 
table  between  the  two.  Look  at  the  picture  (on  page 
41)  and  arrange  the  cards  as  there  represented,  and 
put  the  mirror  in  place  of  the  blackened  glass  on  the 
blocks.  On  looking  through  the  small  hole  in  the 
postal-card  (marked  E  in  the  drawing),  you  will  see 


48  LIGHT. 

in  the  mirror  several  bars  of  yellow  light,  placed  one 
over  the  other.  Again  we  have  an  instance  of  multi- 
plied reflection.  Instead  of  seeing  the  reflections 
thrown  upon  the  wall,  we  can  look  down  upon  them 
and  see  them,  just  as  they  stand,  each  at  its  point  of 
incidence  on  the  glass  and  the  quicksilver.  Study 
these  brilliant  bars  of  light,  examine  the  diagram 
carefully,  and  you  will  readily  see  that  this  experi- 
ment simply  exhibits  in  a  different  manner  the  same 
thing  that  we  saw  in  the  last  experiment. 

EXPERIMENT  WITH  MIRROR  ON  PULSE. 

Get  a  small  bit  of  looking-glass,  about  an  inch  (25 
millimetres)  square,  and  some  wax.  "Warm  the  wax 
in  the  hand  till  it  is  soft,  and  then  make  three  small 
pellets  about  the  size  of  a  pea.  Put  one  of  these  on 
the  back  of  the  little  mirror,  near  the  edge  and  half- 
way between  two  corners.  Place  one  at  each  of  the 
opposite  corners,  so  that  the  mirror  will  have  three 
legs  or  supports  placed  in  a  triangle.  Put  the  helio- 
stat  in  place,  and  bring  a  small  beam  of  sunlight  into 
the  dark  room.  If  this  is  not  convenient,  any  beam 
of  sunlight  in  a  dark  room  (as  in  former  experi- 
ments) will  answer. 

Turn  back  your  coat-sleeve,  and,  while  standing 
near  the  beam  of  light,  place  the  little  mirror  on  the 


REFLECTION  OF  LIGHT.  49 

wrist,  with  one  of  the  wax  legs  resting  on  the  pulse. 
Then  bring  the  arm  into  the  beam,  so  that  the  light 
will  fall  on  the  mirror.  Hold  the  arm  steady,  and 
watch  the  spot  of  reflected  light  thrown  upon  the 
wall.  See  !  It  moves  backward  and  forward  with  a 
curious,  jerking  motion.  It  is  like  the  ticking  of  a 
clock,  or  like  the  beating  of  one's  pulse.  It  is  the  mo- 
tion of  your  pulse.  The  mirror  moves  with  the  pulse, 
and  the  beam  of  reflection  thrown  on  the  wall  moves 
with  it,  and,  though  this  movement  is  very  slight, 
the  reflection  on  the  wall  moves  over  a  space  of  sev- 
eral inches,  and  we  can  see  it  plainly.  In  our  first  ex- 
periment in  reflection  we  learned  that  when  a  mirror 
was  moved  to  the  right  or  left,  the  beam  of  light  re- 
flected from  it  moved  also  to  the  right  or  left,  and  each 
time  through  twice  as  great  an  angle  as  the  mirror. 

This  experiment  is  a  wonderfully  interesting  one, 
and  may  be  tried  with  a  number  of  boys  or  girls,  and 
each  may  see  the  peculiar  beating  of  his  or  her  pulse 
pictured  on  the  wall  in  the  most  singular  and  startling 
manner.  If  any  of  the  persons  whose  pulse-beats  are 
thus  exhibited  get  excited,  laugh  at  the  exhibition, 
or  are  in  any  way  disturbed,  the  change  in  the  move- 
ment of  their  pulse  will  be  quickly  repeated  on  the 
wall,  where  a  hundred  people  can  see  it. 
3 


50  LIGHT. 

EXPERIMENT  WITH  GLASS  TUBE. 

Procure  a  glass  tube,  about  £  inch  (19  millime- 
tres) in  diameter  and  12  inches  (30.5  centimetres) 
long,  and  paint  the  outside  with  black  varnish.  If 
this  is  not  convenient,  cover  the  tube  with  thick 
black  cloth,  and  fasten  it  down  with  mucilage,  taking 
care  to  have  the  cloth  square  at  the  ends.  Punch 

""T\ 

A 


FIG.  11. 


a  hole  in  a  postal-card  with  the  sharp  point  of  a  pair 
of  scissors,  and  with  a  knife  make  the  ragged  edges 
of  the  hole  smooth.  Hold  the  card  at  one  end  of 
the  tube  so  that  the  hole  will  come  just  at  the  centre 
of  the  opening,  and  then,  while  facing  a  window  or  a 
bright  lamp,  look  through  the  tube  with  one  eye,  and 
you  will  see  a  spot  of  light  surrounded  by  a  number 
of  beautiful  rings. 


REFLECTION  OF  LIGHT.  51 

Here  we  have  another  example  of  multiplied  re- 
flection. The  light  entering  the  tube,  through  the 
hole  in  the  card,  falls  on  the  smooth  surface  of  the 
interior  of  the  tube,  and  appears  to  the  eye  in  the 
form  of  rings. 

Fig.  11  represents  a  section  of  the  tube,  and  shows 
the  paths  the  different  rays  of  light  take,  and  shows 
how  each  is  reflected  from  side  to  side  till  they  all 
meet  in  the  eye.  The  dotted  lines  and  the  rings 
projected  beyond  the  tube  show  how  they  appear  to 
the  eye.  By  studying  this  drawing  carefully,  and 
trying  cross  cuts  and  slits  in  the  card  in  place  of  the 
single  hole,  you  will  get  a  very  correct  idea  of  re- 
peated reflection,  and  find  the  tube  a  source  of  con- 
siderable amusement. 

EXPERIMENTS  IN  DISPERSED  REFLECTION. 

Get  a  small  piece  of  black  velvet  or  cloth  and 
take  it  to  a  dark  room,  where  the  heliostat  will  give 
us  a  slender  beam  of  sunlight.  If  this  is  not  con- 
venient, use  a  common  beam  of  sunlight  in  a  dark 
room,  as  in  some  of  our  former  experiments.  Hold 
the  velvet  in  the  hand  between  the  fingers,  and  so  as 
to  leave  the  palm  of  the  hand  clear.  Turn  back  the 
coat-sleeve  so  as  to  expose  part  of  the  white  cuff,  and 
then  bring  the  velvet  into  the  beam  of  sunlight.  You 


52  LIGHT. 

will  observe  nothing  in  particular,  for  the  black,  rough 
cloth  does  not  reflect  the  light  at  all.  Now  move  the 
hand,  so  that  the  spot  of  light  will  fall  on  the  palm. 
See  what  a  pretty,  rosy  glow  of  light  falls  on  the  wall ! 
This  is  the  reflected  light  from  the  hand.  The  skin 
is  rough,  and  the  light  is  diffused  and  scattered  about, 
and,  instead  of  a  bright  spot  of  reflected  light,  as  with 
a  mirror,  we  have  this  glow  spread  all  about  on  the 
wall  and  furniture.  Now  move  your  hand,  so  that 
the  sunlight  falls  on  your  cuff.  Immediately  there  is 
a  bright  light  shining  on  the  wall,  and  lighting  the 
room  with  a  pale,  bluish-white  glare.  Move  the  hand 
quickly,  so  that  the  black  cloth,  the  hand,  and  the 
white  cuff,  will  pass  in  succession  through  the  beam 
of  light.  Observe  how  the  different  things  reflect 
the  light  in  different  degrees.  The  cuff  is  the  smooth- 
est and  whitest,  and  gives  the  brightest  reflection ;  the 
hand  gives  less  light,  because  it  is  less  smooth ;  and 
the  cloth,  that  has  a  very  dark  and  rough  surface, 
gives  no  reflection  at  all,  and  the  spot  of  sunlight 
falling  upon  it  seems  dull  and  faint. 

This  experiment  shows  us  something  more  in  the 
reflection  of  light.  A  piece  of  glass,  the  surface  of 
water,  polished  metals,  ice,  and  all  substances  having 
very  smooth  surfaces,  reflect  light  in  one  direction. 
The  linen  cuff  also  reflected  light,  but  apparently  in  a 


REFLECTION  OF  LIGHT.  53 

very  different  manner  from  the  mirrors  we  have  been 
using. 

Place  a  lighted  lamp  upon  a  table  and  lay  a  mirror 
before  it,  and  you  can  see  a  clear  and  distinct  reflec- 
tion of  the  lamp  and  the  flame  pictured  on  the  glass. 
Put  a  sheet  of  white  paper  before  the  lamp,  and  you 
can  see  only  a  confused  spot  of  reflected  light  on  the 
brightly-lighted  paper.  Lay  a  freshly-ironed  napkin 
or  handkerchief  before  the  lamp,  and  even  the  indis- 
tinct spot  of  light  has  disappeared,  and  the  white 
cloth  reflects  light  equally  from  every  part. 

These  drawings  are  intended  to  show  how  light  is 
reflected  from  different  surfaces.  The  first  represents 
a  smooth  surface,  like  glass,  that  sends  all  the  beams 
in  one  direction,  because  the  points  of  reflection  for 
the  beam  are  in  the  same  plane.  (See  1,  2, 3,  Fig.  12.) 

The  second  drawing  represents  a  slightly-rough- 
ened surface,  like  paper.  Some  of  the  points  of  re- 
flection turn  the  light  one  way,  some  another,  and  the 
beam  of  reflection  is  no  longer  formed  of  parallel 
rays.  They  are  scattered  about,  and  the  image  they 
form  is  confused  and  indistinct.  In  the  third  draw- 
ing we  have  a  rough  surface,  like  cloth,  and  here 
the  rays  of  the  beam  of  reflection  are  scattered  in 
every  direction,  and  we  can  see  no  image. 

It  is  in  this  manner  that  we  are  enabled  to  see  the 


LIGHT. 


FIG.  12. 


people  and  things  about  us.  The  light  of  the  sun  or 
a  lamp  falls  upon  them,  and  is  reflected  into  our  eyes, 
and  we  say  we  see  the  objects.  Very  few  things  re- 


REFLECTION  OF  LIGHT.  55 

fleet  light  so  brightly  that  we  obtain  from  them  a 
reflected  image  of  the  source  of  the  light,  and  we 
generally  see  only  dispersed  and  scattered  light,  that 
does  not  blind  or  dazzle  the  eye,  and  enables  us  to 
look  upon  these  objects  with  ease,  and  to  readily  see 
all  their  parts. 

The  clouds,  the  water,  the  grass,  rocks,  the  ground, 
buildings,  the  walls  inside,  clothing  and  furniture, 
and  everything  we  can  see,  reflect  light  in  every 
direction  again  and  again,  and  thus  it  is  that  all 
spaces,  without  and  within,  are  filled  with  light  so 
long  as  the  sun  shines.  At  night  the  sun  sinks  out  of 
sight,  and  still  it  is  light  for  some  time  after,  for  the 
sunlight  is  reflected  from  the  sunset-clouds  and  the 
sky. 

Sometimes,  upon  a  summer's  day,  when  broken 
clouds  partly  hide  the  sun,  you  will  see  long  bars  of 
dusky  light  streaming  from  openings  in  the  clouds. 
These  long  bars  are  beams  of  sunlight  shining  upon 
dust  and  fine  mist  floating  in  the  air,  and  we  see 
them  because  each  speck  and  particle  reflects  light  in 
every  direction. 

EXPERIMENT  WITH  JAR  OF  SMOKE. 

Fig.  13  represents  a  large,  clean  glass  jar,  such 
as  one  sees  at  the  confectioner's.  It  is  standing 


56 


LIGHT. 


upon  a  black  cloth  laid  upon  a  table  in  a  dark  room, 
and  on  top  of  the  mouth  is  laid  a  postal-card,  hav- 
ing a  slit,  1  inch  (25  millimetres)  long  and  -£$  inch 
(1  millimetre)  wide,  cut  in  it.  Above  the  jar  is  a 


Fio.  13. 


hand-mirror,  so  placed  that  the  beam  of  sunlight  from 
the  heliostat  (or  from  a  hole  in  the  curtain)  will  be 
reflected  downward  upon  the  postal-card  on  top  of 
the  jar. 

This  simple  apparatus  is  designed  to  show  how 


REFLECTION  OF  LIGHT.  57 

light  is  reflected  from  small  particles  floating  in  the 
air.  Set  fire  to  a  small  bit  of  paper  and  drop  it  into 
the  jar.  Place  your  hand  over  the  mouth  of  the  jar, 
and  in  a  moment  it  will  be  filled  with  smoke.  When 
the  paper  has  burned  out,  put  the  postal-card  in  place, 
so  that  the  slit  will  be  in  the  centre  of  the  mouth  of 
the  jar.  Let  the  beam  of  reflected  light  from  the 
mirror  fall  on  this  slit. 

Look  in  the  jar  and  you  will  see  a  slender  ribbon 
of  light  extending  downward  through  the  jar.  Else- 
where it  is  quite  dark  and  black.  Here  we  see  the 
light  streaming  through  the  opening  in  the  card,  and 
lighting  up  the  particles  of  smoke  in  its  path. 

Take  off  the  card,  and  let  the  reflected  beam  fall 
freely  into  the  jar.  The  smoke  is  now  wholly  illu- 
minated, and  the  jar  appears  to  be  full  of  light,  and 
every  part  of  the  bottle  shines  with  a  pale-white 
glow. 

Put  the  postal-card  on  again  and  let  the  light -fall 
through  the  slit.  The  smoke  has  nearly  all  disap- 
peared, and  the  ribbon  of  light  in  the  jar  is  quite  dim. 
Curious  streaks  and  patches  of  inky  blackness  run 
through  it.  What  is  this  ?  Nothing — simply  noth- 
ing. The  smoke  is  melting  away,  and  the  beam  of 
light  disappears  because  there  is  nothing  to  reflect  it 
and  make  it  visible. 


58  LIGHT. 

This  part  of  the  experiment  appears  quite  magi- 
cal in  its  effects,  and  is  exceedingly  interesting. 

THE  MILK-AND-WATER  LAMP. 

Take  away  the  jar  and  put  a  clear  glass  tumbler 
in  its  place.  Fill  this  with  water  and  throw  the 
beam  of  reflected  light  down  upon  it,  and  the  water 
will  be  lighted  up  so  that  we  can  easily  see  the 
tumbler  in  the  dark.  "Now  add  a  teaspoonful  of  milk 
to  the  water  and  stir  them  together.  Throw  the 
beam  of  light  down  once  more.  This  is  indeed  re- 
markable. The  tumbler  of  milk-and-water  shines  like 
a  lamp,  and  lights  up  the  room  so  that  we  can  easily 
see  to  read  by  its  strange  white  light.  Move  the 
mirror  and  turn  aside  the  beam  of  light,  and  instantly 
the  room  becomes  dark.  Turn  the  light  back  again, 
and  once  more  the  glass  is  full  of  light. 

Here  the  minute  particles  of  milk  floating  in  the 
water  catch  and  reflect  the  light  in  every  direction,  so 
that  the  entire  goblet  seems  filled  with  it,  and  the 
room  is  lighted  up  by  the  strange  reflections  that 
shine  through  the  glass. 


REFRACTION  OF  LIGHT.  59 

CHAPTEE  IY. 

REFRACTION    OF   LIGHT. 

CERTAIN  things,  like  glass,  water,  mica,  and  ice, 
allow  light  to  pass  directly  through  their  substance. 
We  hold  them  before  the  eyes,  and  see  the  light  very 
nearly  as  well  as  through  the  air.  Such  substances, 
we  say,  are  transparent.  Other  objects,  like  porce- 
lain or  oiled  paper,  do  not  permit  all  the  light  to 
pass,  and  such  things,  we  say,  are  semitransparent  or 
translucent.  Many  other  things  do  not  permit  light 
to  pass  through  them,  and  cast  shadows  behind  them 
when  brought  into  a  beam  of  light.  These  things 
cut  off  all  the  light,  and  we  call  them  opaque. 

Here  is  a  common  glass  bottle  with  straight  sides 
and  about  three  inches  (76  millimetres)  broad,  or  as 
wide  as  a  postal-card  (Fig.  14).  On  one  side  is  pasted 
a  piece  of  white  paper  having  a  perfectly  round  hole 
cut  in  it.  On  the  glass,  in  the  clear  space  made  by 
the  circular  opening  in  the  paper,  are  two  lines  drawn 
at  right  angles,  in  ink.  These  two  lines  divide  the 
circle  into  four  equal  parts,  and  are  to  serve  as  guides 
in  some  new  experiments. 

Fill  the  bottle  with  clear  water  up  to  the  hori- 


60 


LIGHT. 


Fio.  14. 

zontal  line  in  the  circle,  and  then,  holding  the  bot- 
tle in  a  small  horizontal  beam  of  sunlight  from  the 
heliostat,  you  will  see  that  the  light  passes  directly 
through  the  water  in  the  bottle  or  through  the  air 
above  the  water.  To  make  this  more  distinct,  cut  a 


REFRACTION  OF  LIGHT.  61 

slit,  1^  inch  (38  millimetres)  long  and  -£$  inch  (1 
millimetre)  wide,  in  a  postal-card,  and  place  this 
against  the  side  of  the  bottle,  so  that  the  light  will 
pass  through  the  slit.  This  gives  a  sharp,  clear  beam 
of  light,  and  by  studying  it  carefully,  we  see  that  the 
beam  in  the  air  and  its  continuation  in  the  water 


FIG.  15. 


preserve  the  same  direction.     If  we  place  the  bottle 
on  the  floor  or  table,  and  with  the  mirror  send  a  per- 


62  LIGHT. 

pendicular  beam  down  into  the  water,  we  shall  see 
exactly  the  same  thing. 

Fig.  15  represents  the  bottle  of  water  standing 
upon  a  table,  under  a  window,  where  the  beam  of 
sunlight  enters  from  the  heliostat.  The  opening 
where  the  light  comes  in,  the  mirror,  and  the  re- 
flected beam  of  light  thrown  down  upon  the  bot- 
tle, are  plainly  shown  in  the  picture.  The  postal- 
card  is  held  in  such  a  position  that  the  beam  falls 
upon  the  slit  and  then  enters  the  bottle.  Look  into 
the  bottle  through  the  opening  in  the  paper,  and  see 
where  the  beam  falls,  and  then  move  the  mirror  and 
the  card  till  the  beam  enters  the  bottle  above  the 
water  and  strikes  the  water  just  where  the  two  lines 
meet  in  the  centre  of  the  circle.  Draw  the  postal- 
card  forward  so  that  some  of  the  light  will  cross  the 
outside  of  the  bottle,  and  appears  to  make  a  white 
mark  across  the  paper  circle.  Study  the  two  beams 
outside  and  inside  the  bottle,  and  see  if  you  can  dis- 
cern anything  peculiar  about  them.  The  part  of  the 
beam  inside  the  bottle  and  above  the  water  follows 
the  same  direction  as  the  beam  outside  till  it  touches 
the  water-line,  and  then  it  turns  down  and  takes  a 
new  direction.  This  bending,  that  takes  place  when 
a  beam  of  light  passes  from  air  into  water,  is  called 
refraction.  It  takes  place  very  generally  when  light 


REFRACTION  OF  LIGHT. 


63 


passes  from  one  transparent  medium  to  another,  and 
gives  rise  to  a  number  of  curious  matters  in  regard 
to  light. 

Here  is  a  drawing  of  the  beam  of  light  cross- 
ing the  opening  in  the  paper,  and  showing  'how  it 
is  bent.  It  passes  through  the  air  above  the  water, 


in  the  upper  half  of  the  circle,  and  then  takes  a  new 
direction  through  the  water  in  the  lower  half.  You 
will  observe  in  the  drawing  dotted  horizontal  lines 
extending  from  B  to  J.,  and  from  C  to  Z>.  Look  at 
the  beam  of  light  carefully,  and  with  a  pen  mark 
these  places  A  and  C  on  the  edge  of  the  paper  circle. 
Take  the  bottle  to  the  light  and  measure  off  the  dis- 
tances from  A  to  the  perpendicular  line,  or  along  the 
line  A  B  in  the  drawing,  and  from  C  to  the  perpen- 
dicular line,  or  the  line  C  D  in  the  drawing.  Make 
a  record  of  these  measurements,  and  then  take  the 


64  LIGHT. 

bottle  to  the  dark  room.  Place  it  nearer  to  the  mir- 
ror, and  let  the  reflected  beam  of  light  fall  upon  it 
at  a  different  angle,  being  careful  that  the  beam 
strikes  the  water  at  the  centre  of  the  circle.  Examine 
the  beam  of  light  in  the  bottle,  and  you  will  observe 
that  it  is  bent,  but  at  a  different  angle.  Mark  the 
two  points  where  the  beam  crosses  the  circle  above 
and  below  the  water,  and  measure  their  distances 
from  the  perpendicular  line,  and  then  compare  these 
distances  with  those  we  obtained  the  first  time.  Di- 
vide the  distance  between  A  and  B  by  the  distance 
between  C  and  D,  and  you  will  obtain  a  certain  quo- 
tient. Divide  the  two  sets  of  figures  obtained  the 
second  time  (that  is,  the  distance  from  the  edge  of 
the  circle  to  the  perpendicular  line  above  by  the 
same  below  the  water),  and  the  quotient  or  ratio  of 
the  one  to  the  other  will  be  exactly  the  same  as  be- 
fore. For  instance,  if  the  distance  from  A  to  IB  is 
four  units,  the  distance  from  C  to  D  will  be  three 
units,  and  in  every  experiment  this  proportion  will 
be  the  same.  In  this  case,  where  the  light  passes 
from  the  air  to  the  water,  we  get  a  quotient  that  is 
one  and  a  third,  and  this  quotient  we  call  the  index 
of  refraction.  These  experiments  show  us  that  there 
is  a  fixed  law  of  refraction.  When  the  light  met 
the  surface  of  the  water  at  a  right  angle,  it  passed 


EEFRACTION  OF  LIGHT.  65 

through  the  water  without  bending.  In  such  instances 
the  light  is  said  to  meet  the  water  in  a  normal  direc- 
tion. If  it  meets  the  water  on  either  side  of  this 
normal,  it  is  refracted.  Glass,  diamonds,  mica,  and 
every  transparent  substance,  have  their  own  peculiar 
refraction.  Glass  has  an  index  of  refraction  of  1.5. 
A  diamond  has  quite  another  index  of  refraction, 


FIG.  17. 

and  it  is  by  comparing  these  that  we  are  able-  to 
prove  whether  a  stone  is  a  real  diamond  or  only 
an  imitation  made  of  glass. 

Above  is  a  picture  representing  the  bottle  in  a  new 
position.  The  beam  of  sunlight  enters  the  darkened 
window,  and  falls  upon  a  mirror  lying  flat  on  the 


66 


LIGHT. 


table.  It  is  then  reflected  upward  toward  the  bottle 
that  stands  upon  a  pile  of  books.  The  postal-card 
is  put  up  as  before,  and  the  beam  of  light  passes 
through  the  slit  and  enters  the  bottle  below  the  sur- 
face of  the  water.  Look  at  the  beam  of  light  in  the 
bottle  through  the  circular  opening.  Instead  of  pass- 
ing through  the  water  into  the  air  above  the  surface, 


it  is  bent  and  turns  downward  into  the  water  again. 
If,  at  first,  you  do  not  see  this  curious  effect,  raise  the 
mirror  slightly,  tip  it  up  toward  the  bottle,  and  take 
out  some  of  the  books  under  the  bottle  till  the  beam 
of  light  enters  the  bottle,  in  the  direction  C  0,  as  in 
the  above  drawing.  Here  the  line  A  B  represents 


REFRACTION  OF  LIGHT.  67 

the  surface  of  the  water  in  the  bottle,  and  the  line 
Y  X  is  the  perpendicular  line  in  the  circle.  In 
this  experiment  the  light  must  enter  the  bottle  at  G 
and  pass  to  0  at  the  surface  of  the  water,  and  then 
you  will  see  a  most  curious  phenomenon,  the  reflec- 
tion of  the  light  from  the  surface  of  the  water  at  O 
downward  to  D.  To  understand  this  singular  matter 
we  must  study  the  diagram. 

In  the  diagram  a  beam  of  light  is  represented  as 
entering  the  circle  at  6r,  and  is  then  refracted  to  H. 
Another  beam  goes  from  /  to  J.  Dotted  lines  are 
drawn  from  each  of  these  beams  to  the  perpendic- 
ular, both  above  and  below  the  water.  You  can 
easily  compare  the  relations  of  these  dotted  lines 
above  and  below,  and  you  will  see  that  they  still  pre- 
serve the  same  relation  to  each  other  that  we  dis- 
covered in  former  experiments.  First,  we  must  ob- 
serve that  light  may  pass  from  air  to  water,  as  from 
G  to  0  and  H^  or  from  water  to  air,  as  from  H  to 
0  and  G,  and  the  amount  of  bending  will  be  the 
same  in  both  cases.  In  other  words,  the  light  takes 
the  same  path  in  going  from  air  to  water  as  when 
moving  from  water  into  the  air.  A  beam  of  light 
passing  to  0,  just  above  the  surface  of  the  water, 
will  be  refracted  as  already  described.  To  study 
this  matter  further,  we  must  reverse  the  direction  of 


68  LIGHT. 

the  light  and  cause  it  to  pass  from  the  water  to  the 
air.  The  beam  of  light  entering  at  (7,  below  the 
water,  passes  to  O.  Now,  if  we  measure  a  line  from 
G  to  the  perpendicular  0  X,  we  shall  find  it  is  too 
long  to  be  three  units,  if  we  call  the  length  of  the 
longest  line  (0  B)  that  can  be  drawn  from  the  cir- 
cumference of  the  circle  to  its  centre  four  units.  The 
beam  of  light  entering  the  water  passes  to  the  sur- 
face at  0,  and  finds  itself  a  prisoner,  and  it  turns 
back  and  dives  down  into  the  water  again.  None  of 
our  experiments  have  shown  a  more  singular  result 
than  this.  The  lines  which  we  have  so  often  drawn 
perpendicular  to  the  diameter,  Y  X,  of  the  circle, 
are  called  sines,  and  the  law  of  refraction  is  always 
thus  stated:  The  sines  of  the  angles  of  incidence 
have  a  constant  relation  to  the  sines  of  the  angles 
of  refraction.  In  the  case  of  light  passing  from  air 
into  water,  the  ratio  of  the  sines  is  as  4  to  3  ;  in  the 
case  of  light  passing  from  air  into  glass,  the  ratio  of 
the  sines  is  as  3  to  2. 

The  beam  of  light  entering  the  water  at  G  is  said 
to  have  reached  the  critical  angle  A  O  C,  and  hence 
is  totally  reflected.  By  this  is  meant  that  it  has  gone 
beyond  the  critical  point,  where  the  law  of  the  sines 
comes  to  an  end,  and  reflection  takes  the  place  of  r& 
fraction. 


REFRACTION  OF  LIGHT.  69 

Sometimes,  when  walking  along  a  road  on  a  warm 
day,  you  may  observe  a  curious  quivering  in  the  air 
just  where  the  road  seems  to  meet  the  sky,  as  it  goes 
over  a  hill.  The  objects  near  this  point  appear  to  be 
distorted  and  to  tremble,  or  they  assume  fantastic 
shapes.  Here  we  have  an  instance  of  refraction 
caused  by  the  heated  air  just  above  the  surface  of 
the  road.  The  light  passing  through  these  layers  of 
unequally-heated  air  is  refracted  unequally,  and  the 
objects  that  reflect  the  light  appear  distorted.  In 
some  instances  the  refraction  may  pass  the  critical 
angle,  and  we  may  see  the  objects  apparently  doubled 
by  reflection.  Warm,  calm  days  by  the  sea  show  the 
same  thing,  when  .distant  vessels  appear  repeated  in 
the  sky,  or  when  distant  land  that  is  really  below  the 
horizon  "  looms  "  up  and  glimmers  upon  the  horizon 
in  trembling  headlands.  This  illusion  is  called  the 
mirage,  and  takes  place  when  refraction  exceeds  the 
critical  angle  and  becomes  reflection. 

Fill  a  clear  glass  tumbler  with  water,  and  put  a 
spoon  in  it,  or  dip  one  finger  in  the  water,  and  hold  it 
above  your  head  so  that  you  can  look  into  the  water 
from  below.  You  will  find  that  you  cannot  see  through 
the  water  up  into  the  air  above.  The  under  surface 
of  the  water  will  appear  to  shine  like  burnished  sil- 
ver, and  the  spoon  or  your  finger  will  be  reflected  in 


70  LIGHT. 

it,  as  in  a  beautiful  mirror.  This  illustrates  total 
reflection,  and  shows  that  in  this  case  all  light  thrown 
upward  through  the  water  is  reflected  from  its  sur- 
face. Look  into  the  tumbler  from  above,  and  it  ap- 
pears full  of  clear  water.  Look  into  it  from  below, 
and  it  seems  as  if  an  opaque  sheet  of  silver  rested  on 
the  water,  and  shut  out  the  view  of  everything  above. 
Take  a  small  glass  tube,  and  roll  up  a  piece  of 
colored  paper  or  printed  paper  and  slip  it  inside  the 
tube,  and  then  place  the  tube  in  the  goblet  jof  water. 
Hold  the  goblet  in  the  hand  near  the  eyes,  and  you 
can  see  the  paper  in  the  tube  through  the  water. 
Lower  the  goblet  till  you  can  look  down  into  the 
water  from  above,  and  the  tube-  will  appear  as  if 
made  of  silver,  and  the  paper  will  totally  disappear. 
To  vary  the  experiment,  lift  the  tube  up  and  down 
in  the  water,  and  the  paper  will  appear  and  disappear 
in  the  most  surprising  manner.  This  also  illustrates 
total  reflection.  The  light  reflected  from  the  paper 
passes  through  the  glass  tube  into  the  water,  and  is 
refracted.  In  certain  positions  the  light  passes  the 
critical  angle,  and  is  reflected  from  the  outer  surface 
of  the  glass  tube,  and  fails  to  reach  the  eye.  Look 
into  the  goblet  from  below,  and  there  is  the  colored 
paper  pictured  by  total  reflection  on  the  under  side  of 
the  water. 


REFRACTION  OF  LIGHT. 


71 


THE  WATER-LENS. 


Fig.  19  shows  an  oblong  box  of  pine,  14  inches 
(35.7  centimetres)  high,  6|  inches  (16.5  centimetres) 
square  at  the  outside  at  each  end,  and  made  of  thin 


FIG.  19. 


boards,  nailed  or  screwed  together.  One  side  is  en- 
tirely open,  and  at  the  top  is  a  round  hole,  5  inches 
(12.T  centimetres)  in  diameter.  On  this  opening 
rests  a  hemispherical  glass  dish,  made  by  cutting 


72  LIGHT. 

off  the  round  top  of  a  glass  shade.  This  makes  a 
thin  glass  bowl,  5£  inches  (14  centimetres)  in  diam- 
eter, and  it  rests  in  the  hole,  partly  above  and  partly 
below  the  top  of  the  box. 

Inside  the  box  two  strips  of  wood  are  fastened, 
one  on  each  side,  at  an  angle  of  forty-five  degrees. 
On  these  strips  rests  a  sheet  of  silvered  glass,  5f 
inches  (13.7  centimetres)  wide  and  8^  inches  (21.4 
centimetres)  long,  or  just  large  enough  to  slip  into  the 
box,  as  shown  by  the  dotted  lines  in  the  picture.  To 
keep  the  glass  from  sliding  out,  a  tack  or  brad  may  be 
driven  in  the  box  at  the  end  of  the  mirror. 

Put  the  heliostat  in  the  window,  and  bring  a  full 
beam  of  sunlight  into  the  darkened  room.  Then 
place  this  box  on  the  window-seat,  or  on  a  table  next 
to  the  window,  with  the  open  side  toward  the  window, 
and  in  such  a  position  that  the  beam  from  the  helio- 
stat will  fall  on  the  mirror.  By  this  arrangement  the 
light  will  be  reflected  upward  through  the  glass  bowl. 
Then  fill  the  bowl  with  clear  water,  choosing  the 
purest  and  cleanest  that  can  be  found.  Adjust  the 
box  carefully,  and  see  that  the  beam  from  the  helio- 
stat strikes  the  mirror  fully,  and  that  the  reflected 
beam  meets  the  bowl  on  every  side,  so  that  there  are 
no  shadows  inside  the  box. 

Here  we  have  a  broad  beam  of  light  passing  from 


REFRACTION  OF  LIGHT.  73 

the  air  into  water,  and  our  experiments  have  shown 
us  that  in  such  an  event  the  light  may  be  refracted. 

Hold  a  sheet  of  paper  in  a  horizontal  position 
just  above  the  bowl,  and  you  will  see  that  it  is  fully 
lighted  up  by  the  light  thrown  up  by  the  mirror 
through  the  water.  Raise  the  paper  slowly,  and  the 
circle  of  light  on  the  paper  will  grow  smaller  and 
brighter,  till  it  is  reduced  to  a  small  dot  of  intense 
white  light. 

Put  a  match  just  at  this  bright  spot  of  light,  on 
the  under  side  of  the  paper,  and  instantly  it  begins  to 
burn.  Touch  the  lighted  match  to  the  paper,  and 
hold  the  burning  paper  beside  the  bowl  of  water,  and 
gently  blow  the  smoke  over  the  water.  See  what  a 
strange  cone  of  light  appears  in  the  smoke !  It  is 
pale  below,  next  the  water,  and  grows  brighter  and 
brighter  till  the  top  of  the  cone  is  reached,  and  here 
it  is  intensely  bright.  Above  this  cone  appears  an- 
other, upside  down,  with  its  point  touching  the  point 
of  the  cone  beneath  it.  Above,  on  the  ceiling,  is  a 
large  circle  of  light,  perhaps  several  feet  in  diameter. 

Fig.  20  represents  a  number  of  rays  of  light  en- 
tering at  the  left,  and  reflected  upward  from  the 
mirror.  From  our  experiments  we  learned  that  light 
passing  from  the  air  into  water,  and  reaching  the  sur- 
face in  a  normal  direction,  goes  straight  on  through 


74 


LIGHT. 


the  water  in  the  same  path.    If  it  enters  the  water 
on  either  side  of   this  normal,   it  is  refracted  or 


turned  aside,  and  takes  a  new  path.    The  greater  the 
angle  at  which  it  enters  the  water,  the  greater  the  re- 


REFRACTION  OF  LIGHT.  75 

fraction.  In  the  diagram  the  line  in  the  middle  repre- 
sents the  ray  of  light  in  the  centre  that  meets  the 
water  at  a  normal,  and  passes  straight  through  it 
and  on  into  the  air  above.  On  either  side  the  rays 
are  represented  as  refracted,  or  bent  out  of  their 
track,  and  obliged  to  take  new  paths.  The  greater 
the  distance  of  the  ray  from  the  normal,  the  greater 
its  refraction.  Now,  as  all  the  rays  at  the  same  dis- 
tance from  the  normal  are  refracted  to  the  same  de- 
gree, it  follows  that  there  must  be  a  place  where  all 
these  rays  of  refraction  will  meet. 

Look  at  the  cone  of  light  over  the  bowl  of  water, 
and  you  can  see  the  spot  where  all  the  rays  of  light 
are  concentrated.  Here  they  meet  in  what  is  called  a 
focus.  You  can  readily  remember  this  word,  because 
it  means  a  hearth,  or  turning-place^  and  we  saw  our 
match  take  fire  just  at  that  point.  The  rays  of  sun- 
light contain  heat,  as  well  as  light,  and,  if  we  gather 
them  altogether  in  a  bundle,  of  course  we  shall  con- 
centrate both  the  heat  and  light.  A  bit  of  paper  held 
in  the  focus  glows  with  intensely  white  light,  and 
presently  begins  to  smoke  and  burn  in  the  concen- 
trated heat. 

This  bowl  of  water  is  called  a  lens,  and,  by  means 
of  refraction,  we  may  use  it  to  concentrate  light  and 
heat.  Beyond  the  focus  you  observe  the  light  spreads 


76  LIGHT. 

out  again  till  it  meets  the  ceiling,  where  it  appears 
as  a  broad  disk  of  light.  In  the  diagram  each  ray  is 
represented  as  meeting  at  the  focus,  and  then  all 
pass  each  other  and  go  on  in  their  previous  direc- 
tions; and  you  can  readily  see  that  a  new  cone  of 
light  will  be  formed,  upside  down,  above  the  focus ; 
and  beyond  it  all  the  rays  will  spread  out  wider  and 
wider  the  farther  they  go.  Hold  a  piece  of  paper 
just  above  the  focus,  and  you  will  see  a  small  circle  of 
light  upon  it.  liaise  it  higher,  and  the  circle  grows 
larger  and  larger,  and  on  the  ceiling  it  is  several  feet 
wide. 

By  means  of  lenses  of  glass  or  water  we  can  spread 
out  a  beam  of  light,  gather  a  whole  bundle  of  rays 
into  a  single  focus — we  can  make  distant  objects  ap- 
pear near,  make  small  things  appear  large,  and  large 
things  appear  small.  Telescopes,  microscopes,  specta- 
cles, and  all  kinds  of  optical  instruments,  are  founded 
on  this  simple  law  of  refraction,  as  shown  by  this 
bowl  of  water. 

EXPERIMENTS  IN  PROJECTION. 

At  the  optician's  you  can  purchase  a  small  glass 
plano-convex  lens,  3  inches  (76  millimetres)  in  diam- 
eter, and  of  a  focal  length  of  about  8  inches,  for  per- 
haps less  than  fifty  cents.  Such  a  lens  is  flat  on  one 


REFRACTION   OF   LIGHT. 


77 


side  and  convex  on  the  other,  and  from  this  it  takes 
its  name.  Take  this  lens  into  a  room,  and  close  the 
curtains  at  all  the  windows  save  one.  Soften  a  piece 
of  wax,  and  stick  the  lens  into  it,  so  that  it  will  serve 
as  a  handle,  and  then  hold  the  lens  a  few  inches  from 
the  wall,  or,  if  the  wall  is  dark-colored,  before  a  sheet 
of  paper  pinned  upon  the  wall,  and  just  opposite  the 


FIG.  21. 

window.  On  the  wall  will  then  appear  a  picture 
of  the  window,  and  the  trees,  houses,  and  other 
objects  that  may  be  seen  through  it.  Move  the 
lens  backward  or-  forward,  and  you  will  find  a  place 
where  the  image  on  the  wall  becomes  distinct,  and 
gives  a  miniature  view  of  the  window  in  its  natural 
colors,  and  upside  down. 


78 


LIGHT. 


Here  the  light  from  the  window  falls  upon  the 
lens,  and  is  refracted  to  a  focus.  This  focus  consists 
of  points,  each  of  which  is  formed  by  the  conver- 
gence of  rays  which  come  from  a  similar  point  in 
the  window.  This  we  call  a  projection^  because  the 
light  is  projected  or  thrown  upon  the  wall  by  the  lens. 
To  understand  this  we  must  notice  that  every  part  of 
the  window  sends  light  into  the  room  in  every  direc- 
tion. Every  part  sends  light  into  the  entire  lens,  and 
each  beam  is  refracted  and  takes  a  new  direction  be- 


FIG.  22. 


yond  it,  so  that,  ultimately,  all  the  rays  meet  at  the 
focus.  In  examining  this  projection,  you  will  notice 
that  the  glass  is  quite  near  the  wall  when  the  focus  is 
clear  and  sharp.  If  we  measure  the  distance  from 
the  lens  to  the  projection,  we  shall  get  a  certain  meas- 
urement. This  measure  we  call  the  focal  distance  of 
the  lens. 

Fig.  22  represents  two  rays  from  the  top  of  the 
window  and  two  from  the  bottom,  and   shows  the 


REFKACTION  OF  LIGHT.  79 

path  they  take.  To  draw  every  ray  would  confuse 
the  picture,  and  by  examining  these  four  we  can 
form  an  idea  how  they  all  travel  together  in  a  crowd 
and  meet  beyond  the  lens  in  new  positions,  and  all 
closely  drawn  together  in  a  focus.  Those  from  the 
top  of  the  window  are  refracted  in  one  direction, 
those  from  the  bottom  in  another,  and  thus  they  cross 
each  other,  and  the  projected  image  of  the  window 
appears  to  be  upside  down. 

The  picture  on  page  YV  in  this  section  represents 
the  heliostat  in  position  in  a  dark  room.  On  a  table 
in  front  of  the  instrument  is  the  plano-convex  lens, 
mounted  on  a  lump  of  wax,  fastened  to  a  block  of 
wood,  and  placed  with  the  convex  side  toward  the 
sun.  The  opening  of  the  heliostat  is  covered  by  a 
piece  of  smoked  glass,  having  a  figure  of  an  arrow 
drawn  upon  it.  The  light  passes  through  the  glass 
where  the  smoke  was  brushed  away  in  drawing  the 
arrow,  and  falls  upon  the  lens.  By  refraction  the 
beams  of  light  form  an  image  of  the  arrow  upon  a 
white  screen.  This  screen  is  made  of  white  cotton 
cloth,  and  is  hung  about  15  feet  (4.57  metres)  from 
the  lens.  The  result  is  a  large  projection  of  the 
arrow,  upside  down,  and  in  white  on  a  black  ground. 
Move  the  lens  backward  or  forward  slightly,  and  you 
will  find  a  place  where  the  projection  is  sharp  and 


80  LIGHT. 

clear,  and  then  the  lens  may  be  fixed  there  while  we 
project  other  images  on  the  screen. 

This  simple  and  inexpensive  apparatus  thus  makes 
an  excellent  magic  lantern.  Common  painted  or 
photographic  lantern  -  slides  may  be  placed  upside 
down  at  the  opening  of  the  heliostat,  and  will  be  pro- 
jected on  the  screen  clearly  and  distinctly,  as  with  the 
best  magic  lanterns.  Concerning  the  use  of  this  lan- 
tern, and  the  slides  that  may  be  used  in  it,  more  may 
be  found  under  the  section  on  the  water-lantern. 

If  it  is  not  convenient  to  use  a  heliostat,  this  lan- 
tern may  be  used  by  taking  a  beam  of  sunlight,  as  it 
enters  through  a  hole — 4  inches  (10  centimetres)  in 
diameter — in  the  shutter,  and  reflecting  it  in  a  hori- 
zontal direction  through  the  lens  by  means  of  a  hand- 
mirror. 

THE  FOUNTAIN  OF  FIRE. 

Fig.  23  represents  a  flat-bottomed  flask,  used  by 
chemists.  It  has  a  narrow  neck  at  the  top,  a  flat 
base,  and  a  hole  at  the  side.  This  hole  may  be  cut 
in  the  flask  by  means  of  a  tube  of  brass  one-quar- 
ter inch  in  diameter.  This  tube  has  a  square  end 
which  is  scored  by  two  or  more  cross-cuts  with  a  V- 
shaped  file.  A  block  of  wood,  having  a  hole  of  one- 
quarter  inch  in  diameter,  is  placed  against  the  flask ; 
and  then  the  tube,  armed  with  emery  and  water,  is 


REFRACTION  OF  LIGHT. 


81 


inserted  in  this  hole,  and  by  twirling  the  tube  in  the 
fingers  the  hole  in  the  glass  is  made.  The  tube  may 
also  be  put  in  a  lathe.  In  the  picture  the  flask 
stands  upon  a  shelf  in  front  of  the  heliostat,  and 


FIG.  23. 


just  beneath  it  on  the  floor  is  placed  a  tub  or  a 
water-pail.  These  few  things  and  some  pieces  of 
colored  glass  will  enable  us  to  perform  a  most  inter- 


82  LIGHT. 

esting  and  beautiful  experiment  both  in  refraction 
and  reflection.  Place  the  finger  over  the  hole  in  the 
side  of  the  flask  and  fill  it  with  water.  Place  the 
flask  on  the  shelf  so  that  the  beam  of  light  from  the 
heliostat  will  strike  the  glass  opposite  the  hole  in 
the  side. 

Look  at  the  beautiful  cone  of  light  in  the  water. 
The  beam  of  light  is  refracted  and  brought  to  a  focus 
as  in  our  other  experiments,  except  that  here  the 
cone  is  entirely  under  water.  Study  this  singular  cone 
carefully,  and  adjust  the  flask  so  that  the  point  of 
the  cone  shines  on  the  finger  at  the  hole  in  the  side. 
When  this  is  done,  make  the  room  as  dark  as  possi- 
ble, and  then  remove  the  finger  and  let  the  water  fall 
in  a  stream  into  the  tub  on  the  floor. 

How  magical !  The  curving  stream  of  water  is 
fall  of  light,  and  appears  like  a  stream  of  molten  iron. 
The  spot  where  it  falls  seems  touched  with  fire. 
Put  your  finger  in  the  stream  of  water,  and  it  is 
brightly  illuminated.  Of  course,  the  water  soon  runs 
down,  and  the  display  stops.  To  prevent  this,  bring 
water  in  a  rubber  tube  from  the  water-pipes  in  the 
house,  and  then  regulate  the  supply  so  that  the  re- 
ceiver may  be  kept  full  as  fast  as  the  water  runs  out. 

Place  a  piece  of  red  glass  behind  the  flask  in 
the  beam  of  sunlight,  and  the  stream  of  water  will 


I 

REFRACTION  OF  LIGHT.  83 

look  like  blood.  Touch  it,  and  the  hand  will  be  crim- 
son, and  the  scattered  drops  that  fall  in  a  shower  into 
the  tub  will  shine  like  drops  of  red  fire.  Place  a 
green  or  blue  glass  behind  the  flask,  and  the  stream 
of  water  will  turn  green  or  blue,  and  present  a  most 
singular  appearance.  Hold  a  goblet  in  the  stream, 
and  it  will  overflow  with  liquid  light.  Flashes  and 
sparkles  of  fire  will  appear  in  it,  and  foam  over  the 
sides,  shining  with  brilliant  light. 

This  beautiful  experiment  is  as  interesting  as  it  is 
strange  and  magical,  and  it  illustrates  both  refraction 
and  total  reflection.  The  flask  makes  a  lens,  and 
the  falling  stream  of  water  is  lighted  up  by  the  cone 
of  light  that  enters  it  at  the  hole  in  the  flask.  Both 
the  water  and  the  light  pass  out  of  the  hole  to- 
gether, the  light  inside  of  the  water.  That  this  is  so, 
may  be  proved  by  permitting  the  water  to  escape, 
when  the  light  will  be  seen  shining  out  of  the  hole 
horizontally  into  the  room.  Why,  then,  does  it  not 
shine  out  into  the  room  while  the  water  is  escaping  ? 
When  the  stream  of  water  is  flowing  out,  it  falls  in  a 
curve  into  the  tub  on  the  floor.  The  beam  of  light, 
passing  out  with  the  water,  meets  its  curved  surface 
at  such  an  angle  that  it  is  totally  reflected.  This 
beam  of  reflection  again  meets  the  surface  of  the 
water,  and  is  again  totally  reflected.  In  this  manner 


84  LIGHT. 

it  is  reflected  from  side  to  side,  again  and  again,  till 
it  reaches  the  tub,  and  there  we  see  it  shining  brightly. 
It  is  a  prisoner  in  the  water,  and  follows  it  down  into 
the  tub.  When  you  put  your  hand  in  the  falling 
water,  you  see  that  it  is  lighted  brightly,  and  yet  the 
stream  by  comparison  is  rather  dark.  If  it  were 
pure,  distilled  water,  it  would  hardly  be  visible.  As 
it  is  full  of  floating  specks  and  motes,  each  of  these 
reflects  light,  and  these  cause  the  water  to  appear  full 
of  light. 

This  fountain  of  fire  is  a  charming  experiment 
for  a  school,  and  its  double  lesson  makes  it  as  inter- 
esting as  it  is  beautiful. 

THE  WATER-LANTERN. 

Fig.  24:  represents  the  water-lens  used  in  the 
last  experiment  but  two.  The  water -lens  stands 
in  the  wooden  box  containing  the  mirror,  and  at 
the  back  of  the  box  is  a  wooden  slide  holding  a 
horizontal  shelf  at  the  top.  This  slide  has  a  long 
slot  cut  in  it,  and,  by  means  of  a  bolt  and  nut  fast- 
ened at  the  back  of  the  box,  it  can  be  made  fast  to 
the  box  in  any  desired  position.  This  slide  is  16 
inches  (40.6  centimetres)  long,  5  inches  (12.7  cen- 
timetres) wide,  and  f  inch  (19  millimetres)  thick ;  and 
the  slot  cut  in  it  extends  nearly  the  whole  length. 


REFRACTION   OF  LIGHT. 


85 


FIG.  24. 


shelf  on  the  top  is  7  inches  (17.8  centimetres) 


86  LIGHT. 

long,  and  5  inches  (12.7  centimetres)  wide,  and  has  a 
hole,  3J  inches  (8.3  centimetres)  in  diameter,  cut  in 
the  centre.  The  iron  bolt  and  nut  must  go  through 
the  back  of  the  box,  and  must  have  a  washer  wide 
enough  to  cover  the  slot  in  the  slide.  A  few  inches 
below  the  bolt  a  block  of  wood  is  fastened  to  the 
back  of  the  box  in  the  slot  of  the  slide,  to  serve  as  a 
guide  in  raising  and  lowering  the  slide  carrying  the 
lens.  In  the  hole  in  the  shelf  rests  a  large  watch- 
glass,  or  shallow  dish,  about  4  inches  (10.1  centime- 
tres) in  diameter.  The  plano-convex  lens  used  in  our 
experiments  in  projection  may  be  here  used  in  place 
of  the  watch-glass.  On  each  side  of  the  shelf  are 
two  upright  wooden  arms,  and  between  them  is  placed 
a  looking-glass  7  inches  (17.8  centimetres)  long,  and 
4  inches  (10.1  centimetres)  wide.  To  hold  this  mir- 
ror in  place,  screws  may  be  put  through  the  top  of 
uprights  into  the  frame,  so  that  it  will  hang  sus- 
pended, and  turn  freely  up  or  down. 

This  apparatus  can  be  made  for  about  $3.20,  the 
woodwork  costing  $1,  the  two  mirrors  $1.50,  the  two 
lenses  costing  70  cents ;  and  when  it  is  ready  for  work 
it  will  be  a  fine  lantern  suitable  for  projecting  large 
pictures  upon  a  screen.  •  Place  the  lantern  before  the 
heliostat,  so  that  the  full  beam  of  light  will  be  re- 
flected from  the  mirror  upward  through  the  glass 


REFRACTION  OF  LIGHT.  87 

bowl  and  the  watch-glass.  Fill  each  of  these  with 
clear  water,  and  then  place  the  swinging  mirror  at 
the  top  at  an  angle  of  45°.  Hang  up  a  large  screen 
of  white  cotton  cloth  or  sheet  in  front  of  the  lantern, 
and  from  15  to  40  feet  (4.57  to  12.20  metres)  from  the 
lantern.  On  this  screen  will  appear  a  circle  of  light 
projected  from  the  lantern.  The  sunlight  from  the 
mirror  is  refracted  in  the  large  water-lens  and  brought 
to  a  focus.  It  is  again  refracted  in  the  small  glass  of 
water,  and  is  reflected  by  the  mirror  on  the  screen. 
Get  a  piece  of  smoked  glass,  and  trace  upon  it  some 
letters,  and  then  lay  it  on  the  water-lens  with  the  top 
(upper  side  of  the  writing)  toward  the  screen:  im- 
mediately the  letters  will  appear  on  the  screen,  in 
white  on  a  black  ground.  If  the  projection  is  not 
distinct,  loosen  the  nut  at  the  back  of  the  box,  and 
move  the  wooden  slide  up  or  down  till  the  right  focus 
is  obtained. 

This  water-lantern  may  now  be  used  for  all  the 
work  performed  with  ordinary  magic  lanterns.  Place 
a  sheet  of  clear  glass  over  the  large  lens  to  keep 
the  dust  out  of  the  water,  and  then  lay  common  lan- 
tern-slides on  this  as  in  a  magic  lantern. 

The  most  simple  slides  for  such  a  lantern  can  be 
made  by  laying  thin  paper  over  engravings  or  draw- 
ings, and  tracing  the  picture  with  lines  of  holes 


88  LIGHT. 

pricked  with  a  pin.  In  the  lantern  such  a  paper  slide 
will  show  the  lines  of  the  picture  in  dots  of  light  on 
a  dark  ground.  Another  way  is  to  write  or  draw  on 
sheets  of  smoked  glass.  A  curious  effect  may  be 
made  by  placing  the  smoked  glass  in  the  lantern  and 
writing  upon  it,  upside  down  and  backward,  when  the 
letters  will  appear  to  grow  out  in  big  white  characters 
on  the  dark  screen,  and  afford  much  amusement  to 
all  who  see  it.  Of  course,  the  film  of  smoke  will 
easily  rub  off,  and  each  scratch  and  finger-mark  will 
be  shown  on  the  screen,  and  the  work  is  often  dirty 
and  troublesome ;  but  it  has  the  advantage  of  being 
quickly  done,  and,  if  the  picture  is  not  right,  it  can  be 
rubbed  out  and  another  put  in  its  place. 

A  better  kind  of  slide  may  be  made  by  draw- 
ing with  a  needle  on  sheets  of  gelatine.  Sheets  of 
gelatine,  18  inches  (45.7  centimetres)  square,  can  be 
bought  for  35  cents,  either  pure  and  transparent  or  in  a 
variety  of  colors.  Lay  a  piece  of  this  on  an  engraving, 
and  trace  the  picture,  drawing,  map,  or  outline,  with  the 
point  of  a  large  needle — do  not  press  very  hard  on  the 
gelatine ;  a  mere  scratch  is  enough — and  in  the  lan- 
tern every  line  and  dot  will  be  visible,  in  black  upon 
a  white  or  colored  ground.  To  preserve  these  sheets 
of  gelatine,  put  them  between  sheets  of  glass,  and 
bind  them  together  with  paper  pasted  over  the  edges. 


REFRACTION  OF  LIGHT.  89 

Another  kind  of  slide  may  be  made  by  flowing 
skimmed  milk  over  sheets  of  glass.  When  the  white 
film  of  milk  is  dry,  drawings  may  be  traced  in  it  with 
a  sharp  pencil  or  pointed  stick.  Another  plan  is  to 
rub  Castile  soap  over  glass,  and  to  draw  on  this  in  the 
same  way.  By  this  plan  you  can  destroy  the  picture 
by  rubbing  on  more  soap,  and  you  may  then  make  a 
new  picture  in  it. 

This  lantern  is  quite  as  good  as  the  best  magic 
lanterns.  For  schools,  where  one  boy  or  girl  wishes 
to  show  a  sum  in  arithmetic,  an  example  in  alge- 
bra, a  map,  or  sample  of  penmanship,  to  the  whole 
school,  the  sun-lantern  and  piece  of  smoked  glass,  or 
a  sheet  of  gelatine,  will  enable  him  to  project  it  on 
a  screen,  so  that  a  hundred  boys  and  girls  can  see  it 
at  once. 

Another  interesting  experiment  may  be  made  with 
this  lantern  by  taking  the  glass  cover  off  of  the  large 
lens,  and  dropping  a  very  small  chip  of  wood  in  the 
water.  It  will  be  pictured  in  gigantic  size  upon  the 
screen,  and  curious  fringes  of  shade  will  gather  round 
it,  showing  where  the  water  clings  to  the  wood.  A 
drop  of  camphor  or  of  oil  of  coriander  or  oil  of  cin- 
namon, let  fall  into  the  water,  will  exhibit  geometri- 
cal figures  and  strange  motions  on  the  screen ;  and  a 
few  drops  of  indigo  or  carmine  ink  will  color  the 


90  LIGHT. 

screen  blue  or  red,  and  make  an  excellent  background 
for  some  of  the  pictures. 

To  describe  all  that  could  be  done  with  this  water- 
lantern  and  heliostat  would  fill  a  book.  Having 
made  them,  you  can  consult  other  books  on  making 
projections,  and  find  the  lantern  a  source  of  amuse- 
ment and  instruction  for  hundreds  of  people  for  a 
very  long  time. 

THE  SOLAR  MICROSCOPE. 

Fig.  25  represents  a  common  round  glass  flask, 
about  6  inches  (15.3  centimetres)  in  diameter;  a  com- 
mon pocket-microscope  lens  of  1  inch  focus  (costing 
25  cents),  and  a  glass  slide,  carrying  a  microscopic 
object.  The  flask  is  filled  with  water,  and  is  placed 
on  a  table  just  at  the  opening  of  the  heliostat,  so  that 
the  light  will  be  refracted  in  it  and  brought  to  a 
focus.  It  is  thus  a  water-lens,  and  may  be  used  to 
bring  a  focus  of  light  upon  any  object  placed  near  it. 
Just  behind  this  focus  we  place  a  glass  slide,  contain- 
ing some  object  to  be  examined  in  a  microscope.  To 
hold  this  slide  upright,  we  stick  it  in  a  mass  of  wax. 
The  magnifying-glass  is  fastened  to  a  bit  of  wax  rest- 
ing on  a  block  of  wood,  so  that  it  may  be  moved 
backward  or  forward  along  a  strip  nailed  down  on 
another  block.  About  15  feet  (4.57  metres)  from  the 


REFRACTION  OF  LIGHT. 


91 


table  is  placed  a  screen,  and  on  this  is  projected  a 
large  image  of  the  minute  object  on  the  slide.  A 
cloth  is  hung  over  the  upper  part  of  the  water-lens, 
to  shut  out  the  light,  and  all  other  light  is  excluded 
from  the  room.  This  apparatus  makes  a  solar  micro- 
scope, that  may  be  used  to  project  all  kinds  of  micro- 


FHJ.  25. 

scopic  objects,  so  that  they  can  be  exhibited  before  a 
large  number  of  people.  Tanks  for  holding  animal- 
cules, and  all  objects  used  in  microscopes,  may  be 
placed  in  this  solar  lantern  and  exhibited  upon  a  large 
scale,  with  very  little  trouble,  and  at  only  the  ex- 
pense of  the  flask,  the  pocket-lens,  the  screen,  and 
the  heliostat. 


92  LIGHT. 

CHAPTER  Y. 

TEE  DECOMPOSITION   OF  LIGHT. 

CUT  a  vertical  slit,  an  inch  (25  millimetres)  long 
and  £j  of  an  inch  (1  millimetre)  wide,  in  a  piece  of 
cardboard.  Make  the  slit  with  sharp,  clean  edges, 
and  then  fasten  the  cardboard  over  the  opening  in 
the  heliostat,  and  a  slender  ribbon  of  light  will 
enter  the  dark  room.  In  front  of  this  slit  place  a 
small  block  of  wood,  and  on  this  put  a  lump  of  wax. 
At  the  optician's  you  can  purchase  for  50  cents  a 
good  glass  prism.  Stand  this  upright  in  the  wax,  as 
in  Fig.  26.  Behind  this,  at  a  distance  of  about  15 
feet  (4.57  metres),  hang  up  the  screen  we  used  in  the 
lantern  projections. 

Here  A  is  the  opening  in  the  heliostat,  but  some- 
what exaggerated  in  size.  The  prism  is  at  P,  and  /S 
shows  how  the  screen  is  placed,  but  gives  its  position 
much  too  near  the  prism. 

On  the  screen  will  be  projected  a  band  of  brilliant- 
colored  light,  resembling  the  rainbow.  "We  have  seen 
that  light  may  be  reflected,  and  that  it  may  be  re- 
fracted ;  here  we  discover  that,  by  refraction,  it  may 
be  decomposed — that  a  single  beam  of  white  sun- 


THE  DECOMPOSITION  OF  LIGHT. 


93 


light  may  be  split  into  a  vast  number  of  rays,  each 
having  a  color  of  its  own.  This  beautiful  band  of 
color  is  called  the  solar  spectrum.  Study  it  carefully. 


FIG.  26. 


It  is  quite  impossible  to  count  the  colors,  for  they 
mingle  together  and  merge  into  each  other  by  invisi- 
ble gradations,  so  that  we  cannot  say  where  one  color 


94  LIGHT. 

begins  and  another  ends.  Yet  with  a  little  care  you 
can  make  out  a  number  of  colors,  that  seem  quite  dis- 
tinct. Seven  colors  can  very  easily  be  counted  by 
beginning  at  the  red,  or  left  end  of  the  spectrum. 
These  colors  are  red,  orange,  yellow,  green,  blue,  in- 
digo, and  violet.  Some  people  count  ten,  and  call 
them  red,  orange,  yellow,  yellowish-green,  green, 
bluish -green,  ultramarine -blue,  indigo,  violet,  and 
lavender.  All  these  colors,  and  the  countless  shades 
of  colors  that  lie  between  them,  are  in  the  beam  of 
sunlight.  Decompose  the  light  by  means  of  a  prism, 
and  they  stand  side  by  side  on  the  screen  in  a  beauti- 
ful band  or  belt.  Each  color  has  a  different  degree 
of  refraction,  the  refraction  increasing  from  the  red 
to  the  violet ;  and  thus  they  meet  the  screen  at  differ- 
ent places,  and  we  see  them  spread  out  side  by  side 
like  a  band  or  ribbon  upon  the  screen. 

To  prove  that  this  solar  spectrum  is  the  solar 
light  decomposed,  and  to  show  that  all  these  colors 
may  be  found  in  a  beam  of  white  light,  place  a  hand- 
mirror  in  the  beam  of  refracted  light  just  beyond  the 
prism.  The  spectrum  may  thus  be  reflected  to  a  dis- 
tant part  of  the  room,  on  the  wall  or  ceiling.  Then, 
holding  the  mirror  in  the  fingers,  make  it  vibrate  to 
and  fro,  so  that  the  reflected  spectrum  will  move,  in 
the  direction  of  its  length,  from  side  to  side  very 


THE  DECOMPOSITION  OF  LIGHT.  95 

quickly.  At  once  the  spectrum  on  the  wall  changes 
into  a  streak  of  white  light,  with  colored  spots  at 
each  end.  To  understand  this,  you  must  remember 
the  common  experiment  of  whirling  a  lighted  stick 
or  bit  of  live  coal.  The  spot  of  fire  changes  into  a 
ring  of  light.  When  light  falls  upon  the  eye  its  effect 
lingers  for  a  short  time,  even  after  the  source  of  the 
light  has  moved  away,  or  has  ceased  to  give  light. 
The  vision  is  said  to  persist  or  stay  after  the  light  has 
really  gone.  So  in  this  case  the  colors  of  the  moving 
spectrum  on  the  wall  persist  or  stay  in  the  eye  while 
they  are  moving  to  and  fro,  and  thus  one  color  over- 
laps another,  and  we  seem  to  see  them  all  at  once  in 
one  place.  This  mingling  of  every  color  in  the  eye 
gives  us  a  band  or  streak  that  appears  white,  and 
thus,  indirectly,  proves  that  all  the  colors  of  the 
spectrum  make  white,  and  that  white  light  contains 
all  these  colored  lights.  At  the  ends  of  this  band  of 
white  are  bright  spots  of  color.  As  the  mirror  moves 
backward  and  forward,  it  stops  at  each  end  of  its  lit- 
tle journey  to  change  its  direction,  and  here  the  spec- 
trum becomes  visible. 

EXPERIMENTS  WITH  THE  SOLAR  SPECTRUM. 

Send  to  the  dealer  in  artists'  materials  and  get  a 
cake  of  red  vermilion,  emerald-green,  and  aniline  vio- 


96  LIGHT. 

let  (Hoffman's  violet — B.  B.).  If  this  color  cannot  be 
found,  buy  "  Nuremberg  violet."  Get  these  shades, 
and  no  others,  and  then  cut  out  three  narrow  strips  of 
cardboard,  and  give  one  a  coat  of  the  red,  one  a  coat 
of  violet,  and  paint  the  other  green.  Take  pains  to 
give  them  a  good  thick  coat,  so  as  to  hide  the  white 
paper.  Study  the  solar  spectrum  on  the  screen  care- 
fully, and  you  will  see  that  these  shades  of  red,  green, 
and  violet,  are  in  it.  When  the  painted  strips  are 
dry,  take  the  red  vermilion  strip  and  hold  it  in  the 
spectrum  at  the  left  or  red  end,  and  you  will  see  that 
it  matches  the  red  exactly.  Tip  the  paper  backward  a 
trifle  so  that  the  surface  of  the  paper  will  not  shine  or 
glisten  in  the  light,  and  then  move  it  slowly  to  the 
right,  keeping  it  before  the  spectrum.  As  it  passes  the 
orange  it  grows  dark ;  in  the  yellow  it  is  darker  still ; 
opposite  the  green  it  is  perfectly  black.  Move  it  to  the 
very  end,  and  everywhere  the  red  strip  is  quite  black. 
Place  it  before  the  red  again,  and  its  color  comes  out 
clear  and  bright.  Try  the  violet  strip  in  the  same 
way.  In  the  same  manner,  the  green  strip  is  green 
when  it  is  in  the  green  part  of  the  spectrum,  and  black 
everywhere  else. 

This  experiment  shows  that  green,  red,  arid  violet, 
are  visible  in  green,  red,  and  violet  light,  and  that 
in  light  of  any  other  color  they  are  invisible,  and  the 


THE  DECOMPOSITION  OF  LIGHT.  97 

strip  of  card  appears  to  be  black.  Hence,  an  object 
appears  of  its  proper  color  because  it  absorbs  all 
colors  of  the  white  light  except  its  own  color  which 
it  reflects. 

Look  at  the  spectrum  closely,  and  you  will  notice 
that  the  red  is  at  one  end,  the  green  near  the  mid- 
dle, and  violet  is  at  the  other  end.  Between  the 
red  and  the  green  you  will  notice  many  shades 
of  yellow,  from  deep-orange  to  yellowish-green, 
and  between  the  green  and  the  violet  are  many 
shades  of  blue,  from  greenish-blue  to  deep  indi- 
go. 

It  is  thought  that,  when  we  see  a  red  light,  cer- 
tain nerves  in  the  eye  are  affected,  and  convey  a 
peculiar  sensation  to  the  brain,  that  we  call  Ted. 
These  nerves  are  sensitive  to  red  light,  but  are  not 
sensitive  to  any  other  light,  except  in  a  moderate 
degree.  Another  set  of  nerves  in  the  eye  are  pecul- 
iarly sensitive  to  green  light,  and  still  another  set  are 
affected  by  violet  light.  Hence  the  sensations  caused 
by  these  three  colors  are  called  the  three  elementary 
color  sensations,  and  from  the  combinations  of  these 
sensations  come  all  countless  shades  of  color.  "When 
one  of  these  colors  falls  on  the  eye,  we  see  it  dis- 
tinctly. When  two  —  say  the  red  and  green  — 

meet  the  eye,  both  sets   of  nerves  are   affected   at 
5 


98  LIGHT. 

once,  and  we  get  a  sensation  that  is  neither  red  nor 
green,  but  yellow.  In  the  same  manner,  when  green 
and  violet  meet  in  the  eye,  the  two  sets  of  nerves  are 
excited,  and  we  see  not  green  and  violet,  but  blue. 
In  the  same  manner,  if  red,  green,  and  violet  light 
enters  the  eye,  all  the  nerves  are  excited  at  once,  and 
we  see  not  three  colors,  but  one,  which  is  white. 

Purple. 


Bed. 


Violet. 


This  diagram  will  assist  us  to  remember  the  rela- 
tion the  color  sensations  bear  to  each  other.  The 
red  and  the  green  combine  to  make  yellow ;  the  green 
and  the  violet  unite  to  make  blue  ;  all  three  mingled 


THE  DECOMPOSITION  OF  LIGHT.  99 

together  give  us  white.     We  may  also  combine  red 
and  violet  light,  and  get  purple  light. 


THE   COLOR-TOP. 


This  picture  represents  a  common  iron  top,  that 
may  be  found  at  the  toy-shops.     If  you  cannot  find 


Fro.  28. 


one  exactly  like  it,  there  are  others  having  a  straight 
handle  at  the  top,  instead  of  the  curved  handle. 


100  LIGHT. 

Just  under  the  flat  part  of  the  disk  are  two  or 
three  round  pieces  of  drawing-paper,  and  under  these 
is  a  thick  disk  of  pasteboard.  Each  of  these  has  a 
hole  in  the  centre,  so  that  it  can  be  slipped  over  the 
leg  of  the  top.  In  some  tops,  however,  it  may  be 
easier  to  put  these  disks  above  on  the  handle.  Such 
a  top  as  this  may  be  made  to  spin  in  a  dinner-plate 
on  the  table  by  winding  a  string  round  the  leg,  and 
then  pulling  it  away  with  the  right  hand,  while  the 
top  is  held  upright  by  the  left  hand. 

Get  some  thick  drawing-paper,  and  cut  out  three 
disks,  each  4  inches  (10  centimetres)  in  diameter,  and 
make  a  hole  in  the  centre  of  each,  so  that  it  will  slip 
over  the  leg  of  the  top.  Cut  each  disk  open  from  the 
circumference  to  the  centre  with  a  pair  of  scissors. 
Paint  one  with  the  red  vermilion,  one  with  the 
emerald-green,  and  the  other  with  the  violet  that  we 
used  in  the  last  experiment,  and  then,  while  these  are 
drying,  make  a  disk  of  thick  pasteboard,  and  cut  a 
hole  in  the  middle,  so  that  it  will  slip  tightly  over  the 
leg  of  the  top. 

"When  these  are  ready,  take  the  red  and  green 
disks  and  hold  them  side  by  side  with  the  cut  places 
opposite,  and  slip  one  into  the  other,  and  then  turn 
them  round,  so  that  the  green  covers  the  red.  Then 
put  them  both  on  the  leg  of  the  top,  and  put  the 


THE  DECOMPOSITION  OF  LIGHT.  101 

pasteboard  disk  under  them,  to  hold  them  in  place. 
Now,  if  you  hold  the  top  upright  in  a  plate  and  make 
it  spin,  you  will  see  a  beautiful  ring  of  green  color 
round  the  spinning  top. 

When  it  stops,  take  off  the  pasteboard,  and  revolve 
the  colored  disks  one  on  the  other,  so  that  half  of 
the  red  and  half  of  the  green  can  be  seen.  Now  spin 
them  on  the  top,  and  instantly  you  have  a  ring  of 
yellow.  Move  the  disks  again,  so  as  to  display  one- 
quarter  of  the  green  and  three-quarters  of  the  red, 
and  when  the  top  spins  you  get  a  deep-orange  ring. 
Move  them  again,  and  let  the  green  hide  nearly  all  the 
red,  and  the  top  shows  a  greenish-yellow  ring.  In  this 
manner  you  may  mix  the  red  and  green  in  greater 
or  less  proportions,  and  the  ring  of  color  on  the  top 
will  exhibit  new  shades  of  yellow  with  every  change. 

In  the  same  way,  combine  the  green  disk  and  the 
violet,  and  the  spinning  top  will  show  a  new  shade  of 
blue  with  every  proportion  in  which  the  green  and 
violet  are  mixed.  Put  on  the  red  and  violet  disks, 
and  let  each  show  more  or  less,  and  shades  of  purple 
will  be  shown.  Put  on  all  three  disks — the  red, 
green,  and  violet — and  arrange  them  so  that  one-third 
of  each  is  shown,  and  the  ring  will  be  gray.  Change 
the  proportions,  and  you  will  see  each  time  new 
shades  of  gray  or  white. 


102  LIGHT. 

This  is  a  very  simple  toy,  but  it  serves  to  show 
how  these  three  colors  may  be  combined  to  produce 
every  color  in  the  solar  spectrum.  The  color  will 
vary  very  greatly,  and  new  and  beautiful  shades  of 
yellow,  blue,  purple,  and  gray,  will  be  found  at  every 
trial.  Red,  green,  and  violet,  may  be  tinted  with 
other  colors  in  the  most  charming  manner,  and  hours 
can  be  filled  with  amusement  and  instruction  by  ex- 
perimenting with  this  color-top  and  its  ever-chang- 
ing colored  rings. 

To  exhibit  the  colored  rings  on  this  top  before  a 
number  of  people,  make  a  disk  of  stiff  cardboard  about 
5  inches  (12.7  centimetres)  in  diameter,  and  cut  out  three 
holes  at  equal  distances  from  each  other  near  the  edge. 
Over  these  holes  place  pieces  of  red,  green,  and  violet, 
or  ultramarine-blue  glass,  one  color  on  each  hole,  and 
fasten  them  down  with  little  bands  of  paper  at  the 
edges,  and  secured  with  mucilage.  Place  this  disk  on 
the  color-top,  and  hold  it  upside  down  just  above  the 
large  lens  in  the  water-lantern.  Have  the  lantern 
prepared  to  give  projections  on  the  screen  (see  sec- 
tion on  water-lens),  and  then  you  will  see  three 
spots  of  colored  light  on  the  screen,  and,  by  making 
the  top  spin  round  on  the  handle  by  means  of  the 
string,  the  three  spots  of  color  will  whirl  round  in 
a  ring,  and,  if  the  top  moves  fast  enough,  we  shall  see 


THE  DECOMPOSITION  OF  LIGHT.  103 

a  ring  of  white  or  gray.  Cover  the  violet  glass  so  as 
to  shut  out  all  the  light,  and  then  make  the  top  spin, 
and  the  two  spots  of  red  and  green  will  appear  on  the 
screen  in  the  form  of  a  yellow  ring.  In  this  manner 
all  the  effects  exhibited  by  the  color-top  may  be  pro- 
jected on  a  large  scale  on  the  screen,  and  make  a  most 
interesting  and  beautiful  exhibition  that  will  be  sure 
to  please  all  who  see  it. 

DIRECT    RECOMPOSITION    OF    THE    COLORS    OF    THE 
SPECTRUM. 

Let  the  spectrum  fall  on  a  mirror,  and  throw  its 
reflection  upon  a  distant  part  of  the  room.  Procure 
a  slip  of  looking-glass  half  an  inch  wide  and  about 
three  inches  long,  and  place  this  on  the  mirror  in  any 
color  of  the  spectrum.  By  tilting  the  slip  of  looking- 
glass,  any  color  can  be  thrown  on  to  any  other  color 
of  the  spectrum,  and  thus  an  endless  variety  of  colors 
can  be  formed  by  compounding  their  elementary  com- 
ponent colors. 

EXPERIMENTS  IN  REFLECTED  COLORS. 

Fig.  29  represents  a  flat  block  of  wood  having  a 
short  stick  set  up  at  one  corner.  On  this  stick  is 
fastened,  by  means  of  a  lump  of  wax,  a  strip  of  clear 
window -glass  about  1  inch  (25  millimetres)  wide, 


104 


LIGHT. 


and  3  inches  (7.5  centimetres)  long.  Just  behind  the 
stick  is  another  piece  of  glass  of  the  same  size  fast 
ened  to  the  block  of  wood  by  a  mass  of  -wax.  Place 
the  instrument  on  a  table  near  a  window,  and  then  sit 
before  it  with  your  back  to  the  light.  Cut  out  small 


Fm.  29. 

bits  of  paper  and  paint  one  red-vermilion,  anoth- 
er emerald-green,  and  the  third  violet,  as  in  our 
experiments  with  the  color-top.  Then  place  one 
of  these  in  front  of  the  instrument,  at  the  spot 
marked  C  in  the  drawing,  and,  sitting  close  to  the 
instrument,  look  down  into  the  glass  on  the  stick 
and  you  will  see  the  bit  of  colored  paper  reflected  in 
the  glass.  Suppose  this  is  the  red  piece.  Then  place 
the  green  piece  at  the  ring  marked  A.  On  looking 


THE  DECOMPOSITION  OF  LIGHT,  1Q5 

into  the  glass  you  can  now  see  both  the  green  piece 
reflected  in  the  glass  and  the  red  behind  it.  While 
thus  looking  at  both,  move  the  one  or  the  other  till 
they  appear  to  come  in  line,  or  one  over  the  other, 
and  then,  in  place  of  seeing  a  red  and  a  green  piece, 
you  will  see  a  single  yellow  piece. 

Again  we  have  a  combination  of  colors,  and  we 
can  place  the  red  and  violet  or  the  violet  and  green 
pieces  before  and  behind  the  glass,  and  see  the  colors 
combine  precisely  as  in  the  color-top.  If  it  is  not 
convenient  to  make  this  instrument,  these  effects  can 
be  shown  with  a  piece  of  clear  window-glass,  by 
simply  holding  it  in  the  hand  so  that  one  color  can 
be  seen  reflected  in  the  glass,  and  the  other  directly 
through  it. 

With  the  instrument  we  can  combine  all  three 
colors  by  placing  them  in  the  positions  marked  A,  B, 
and  Oy  and  then  looking  through  both  glasses  at  once. 
The  color  at  A  will  be  seen  through  both  glasses,  the 
color  at  C  will  be  seen  in  the  upper  glass  and  in  a  line 
with  the  first,  and  the  color  at  B  will  be  seen  reflected 
on  the  surface  of  the  lower  glass;  and,  if  all  three 
are  in  the  right  places,  we  shall  see  only  one  piece, 
and  that  will  be  white  or  gray. 


106  LIGHT. 

EXPERIMENTS  IN  CONTRASTED   COLORS. 

Cut  out  three  pieces  of  drawing-paper  about  2 
inches  (5.1  centimetres)  square,  and  paint  one  red, 
another  green,  and  another  violet,  using  the  paints 
we  bought  for  the  color-top.  If  these  are  not  at 
hand,  cut  out  squares  of  red,  green,  and  violet  paper, 
and  squares  of  yellow,  pink,  blue,  or  any  other  colors 
you  can  obtain.  Lay  a  piece  of  black  cloth  on  a  table, 
near  the  window,  and  then  sit  before  it  with  your 
back  to  the  light,  and  place  the  red  square  of  paper  on 
the  cloth.  Take  a  sheet  of  white  or,  still  better,  light- 
gray  blotting-paper  in  the  right  hand,  and  hold  it  just 
above  the  red  square  in  such  a  position  that  it  can  be 
quickly  slipped  over  it,  so  as  to  hide  it  from  sight. 
ISTow  look  steadily  at  the  red  square  for  a  minute  or 
two,  and  then  slip  the  gray  paper  over  it.  In  a  few 
seconds  there  will  appear  on  the  gray  paper  a  curious 
image  just  the  size  and  shape  of  the  red  square,  but 
of  a  bluish-green  color.  It  will  grow  brighter  quickly, 
and  then  fade  away,  leaving  nothing  but  the  gray 
paper.  Put  the  green  square  on  the  black  ground, 
and,  after  looking  at  it  for  a  moment,  cover  it  with 
the  gray  paper,  and  a  pink  image  of  the  square  will 
seem  to  shine  out  of  the  gray  paper  for  a  moment, 
and  then  fade  away.  Look  at  the  violet  square,  and 


THE  DECOMPOSITION  OF  LIGHT.  1Q7 

then  suddenly  hide  it,  and  a  pale,  greenish-yellow 
image  will  be  seen.  Get  a  square  of  yellow  paper, 
and  repeat  the  experiment,  and  you  will  see  a  violet- 
blue  image.  In  the  same  way  try  an  orange  square 
and  get  a  violet  image ;  try  greenish-yellow,  and  get 
a  pink  image. 

These  strange,  ghostly  after-images  that  linger 
after  any  color  has  been  suddenly  removed,  result 
from  an  action  in  the  eye.  Having  looked  at  red  till 
the  eye  becomes  weary,  and  then  having  suddenly 
taken  the  red  away  and  replaced  it  by  a  white  surface, 
the  nerves  of  the  eye  send  us  another  sensation  that 
we  call  bluish-green.  The  nerves  sensitive  to  red 
having  become  fatigued,  the  nerves  sensitive  to  green 
and  violet  are  fresh  and  fully  sensitive  to  the  green  and 
violet  rays  in  the  white  light  reflected  from  the  paper. 

Every  color  gives  a  particular  after-image,  and  this 
image  is  always  of  a  color  that  is  said  to  be  comple- 
mentary to  it.  Eed  is  complementary  to  bluish-green, 
orange  to  sky-blue,  yellow  to  violet-blue,  green  to 
pink,  and  so  on  through  all  the  colors. 

To  vary  this  experiment,  you  may  take  a  small  bit 
of  green  paper,  say  about  the  size  of  a  wafer,  and  lay 
it  on  the  middle  of  a  square  of  orange-colored  paper, 
and  then,  after  looking  at  the  two  colors  for  a  mo- 
ment, hide  them  both  with  the  gray  paper,  and  the 


108  LIGHT. 

after-image  will  be  blue,  with  an  orange  spot  in  the 
centre.  Take  off  the  green  wafer  and  put  on  a  blue 
one,  and  make  the  experiment.  Put  a  large  spot  of 
ink  in  the  centre  of  this  orange  square,  and  the  blue 
after-image  will  have  a  gray  spot  in  the  centre.  Black, 
we  know,  is  the  absence  of  color,  and  on  looking  at 
the  black  spot  the  eye  is  not  excited  at  all,  and  the 
blue  image  appears  to  have  a  hole  in  the  middle 
through  which  we  see  the  gray  paper.  If  we  put 
a  white  wafer  on  the  orange  square,  we  shall  see  a 
blue  after-image  with  an  orange  spot. 

These  strange  and  curious  after-images,  appearing 
like  colored  ghosts  on  the  gray  paper,  may  afford  both 
amusement  and  instruction,  and  will  give  us  the  com- 
plementary color  of  any  color  we  use  in  the  experi- 
ments. These  complementary  colors,  when  placed  side 
by  side,  always  give  the  eye  a  pleasing  sensation,  and 
we  say  that  the  colors  look  well  together. 

Take  a  piece  of  red  cloth  or  paper  and  hang  it  up 
before  a  white  screen,  or  upon  a  white  wall,  in  a  dark 
room.  Stand  near  the  window  and  look  at  the  red 
cloth,  and  you  will  not  be  able  to  see  it.  Then 
slowly  open  the  window-shutter,  and  permit  a  little 
light  to  enter  the  room.  Now  the  red  cloth  looks 
like  a  black  patch  on  a  gray  wall.  Let  in  a  little 
more  light,  and  it  turns  to  a  deep,  dark  red.  Gradu- 


THE  DECOMPOSITION  OF  LIGHT.  109 

ally  let  in  more  and  more  light,  and  the  deep-red 
cloth  will  change  to  a  lighter  and  lighter  shade, 
till  the  room  is  fully  lighted,  when  it  will  appear 
in  its  real  color.  Bring  the  red  cloth  into  the  full 
sunlight,  or  throw  a  beam  from  the  heliostat  upon  it, 
and  it  assumes  a  still  lighter  shade  of  red.  Close 
the  shutters  again,  and  it  will  change  back  from 
red  to  dark-red,  and  through  every  shade  to  per- 
fect black.  Try  any  other  color  in  the  same  man- 
ner, and  you  will  produce  precisely  the  same  effects. 
This  experiment  may  be  tried  in  the  night  by  turning 
the  gas  (or  other  lamp)  slowly  down  till  the  light  dis- 
appears, and  turning  it  up  again,  while  you  watch  the 
changing  shades  of  color  as  the  light  decreases  and 
increases. 

Here  we  have  quite  a  different  matter,  showing 
that  the  shade  or  brightness  of  a  color  depends  upon 
the  amount  of  light  it  receives.  If  it  has  plenty  of 
light,  it  appears  of  its  normal  shade ;  if  it  has  less 
light,  it  takes  a  darker  shade;  if  more,  it  has  a 
brighter  shade.  If,  in  using  our  color-top,  we  put  a 
bit  of  black  cloth  or  paper  on  the  colors,  the  ring  of 
color,  when  the  top  spins,  will  take  a  darker  shade. 
If  we  put  on  a  piece  of  white  paper,  we  shall  get 
rings  of  a  lighter  shade.. 

When  the  sunlight  falls  on  any  object,  the  object 


110  LIGHT. 

absorbs  part  of  the  light  and  reflects  the  rest.  If  it 
absorbs  all  the  light  and  reflects  nothing,  the  eye  sees 
no  reflected  rays,  and  we  say  the  object  is  black,  or  is 
invisible.  If  it  reflects  all  the  light,  all  colors  enter 
the  eye,  and  we  say  it  is  white.  If  it  absorbs  all  the 
rays  of  the  spectrum  except  red,  our  eyes  receive 
these  red-reflected  rays,  and  we  call  the  object  red. 
If  it  absorbs  all  the  rays  except  red  and  green,  the 
eye  receives  these  two  rays,  two  sets  of  nerves  are 
excited  at  once,  and  we  say  that  the  object  is  yellow. 
It  is  in  this  manner  that  we  see  the  things  about  us, 
and  are  enabled  to  recognize  the  colors  in  which  they 
appear  to  be  clothed. 


CONCLUSION.  HI 


CONCLUSION. 

have  now  seen  how  light  moves  through  air, 
water,  and  other  transparent  substances;  we  have 
learned  something  of  the  manner  in  which  it  may  be 
reflected  and  refracted  ;  and  we  have  examined  a  few 
of  the  more  simple  facts  about  colors.  Yet  we  have 
not  by  any  means  learned  all  that  is  known  about 
light,  nor  have  we  exhausted  half  the  capabilities 
of  our  apparatus.  We  have  studied  reflection  from 
plane  surfaces  :  all  the  wonderful  effects  produced  by 
reflection  from  curved  mirrors  remain  for  further 
study.  We  have  examined  only  one  or  two  of  the 
different  kinds  of  lenses ;  and  in  the  beautiful  science 
of  colors  we  have,  as  it  were,  only  opened  the  gate 
into  a  strange  and  marvelous  country. 

You  might  go  on  for  a  year  and  still  make  experi- 
ments every  day,  and  even  then  not  reach  the  end. 
You  have  seen  that  it  is  not  difficult  to  make  experi- 
ments ;  and,  should  you  take  up  other  books  on  light 
and  make  new  experiments,  you  would  find  much 
that  would  be  of  the  greatest  value  and  interest. 

Should. you  learn  nothing  else,  you  will  see  for 
yourself  with  what  skill,  wisdom,  and  goodness,  all 


112  LIGHT. 

these  beneficent  laws  Lave  been  arranged.  These 
things  came  not  by  chance,  or  of  themselves.  They 
all  point  to  a  great  and  wise  Creator,  who  has  given 
the  light  a  pathway,  and  filled  it  with  bewildering 
and  perpetual  beauty.  It  is  the  light  that  paints  the 
flowers,  tints  the  clouds,  and  decks  the  sky  in  blue. 
Everything  selects  its  own  particular  color  out  of  the 
solar  spectrum,  and  shines  with  all  the  beauty  and 
glory  of  the  light.  No  man  hath  counted  all  the 
glories  of  light,  nor  hath  any  man  yet  traced  all  its 
paths.  It  brings  us  strange  messages  from  distant 
suns ;  it  makes  all  Nature  beautiful. 

Having  made  a  fair  start  in  the  art  of  experi- 
menting, let  us  go  on  to  new  experiments  in  sound, 
heat,  magnetism,  electricity,  and  mechanics. 


THE  END. 


List  of  Apparatus  used  in  the  Experiments  on  Light, 
with  the  prices,  as  supplied  by  Samuel  Hawk- 
ridjje,  successor  to  George  Wale  §•  Co.,  Hobolcen, 
New  Jersey. 

Heliostat $6  00 

Water-lantern 6  00 

Glass  tube 05 

Pulse-mirror 05 

Square  bottle  for  refraction 15 

Plano-convex  lens 75 

Small  double-convex  lens 50 

Flask  for  condenser  of  solar  microscope 75 

Glass  cylinder  for  experiment  of  the  illuminated  jet — to  be  used 
with  plano-convex  lens  in  place  of  the  large  flask  shown  in 

Fig.  23 1  50 

Glass  prism 50 

Top 10 

Cake  of  vermilion  paint 15 

Cake  of  emerald  green  paint 15 

Nuremberg  violet,  in  powder,  to  be  used  with  gum-water 15 

Two  small  slips  of  clear  glass 05 

$14  85 


LIGHT: 


A   SERIES    OF    SIMPLE,    ENTERTAINING,  AND   INEXPENSIVE   EX- 

PERIMENTS  IN  THE  PHENOMENA  OF  LIGHT,  FOR  THE 

USE  OF  STUDENTS  OF  EVERY  AGE. 

By  ALPKED  M,  MAYEB  and  CHARLES  BABNABD. 

Price,  $1.00. 


From  the  New  York  Evening  Post. 

"  A  singularly  excellent  little  hand-book  for  the  use  of  teachers,  parents,  and  chil- 
dren. The  book  is  admirable  both  in  design  and  execution.  The  experiments  for 
which  it  provides  are  so  simple  that  an  intelligent  boy  or  girl  can  easily  make  them,  and 
so  beautiful  and  interesting  that  even  the  youngest  children  must  enjoy  the  exhibition, 
while  the  whole  cost  of  all  the  apparatus  needed  is  only  $15.00.  The  experiments  here 
described  are  abundantly  worth  all  that  they  cost  in  money  and  time  in  any  family 
where  there  are  boys  and  girls  to  be  entertained." 

Front  the  New  York  Scientific  A  merican. 

"  The  experiments  are  capitally  selected,  and  equally  as  well  described.  The  book 
is  conspicuously  free  from  the  multiplicity  of  confusing  directions  with  which  works  of 
the  kind  too  often  abound.  There  is  an  abundance  of  excellent  illustrations." 

Front  the  A  nterican  Journal  of  Science  and  A  rts. 

"The  experiments  are  for  the  most  part  new,  and  have  the  merit  of  combining  pre- 
cision in  the  methods  with  extreme  simplicity  and  elegance  of  design.  The  aim  of  the 
authors  has  been  to  make  their  readers  '  experimenters,  strict  reasoners,  and  exact  ob- 
servers,' and  for  the  attainment  of  this  end  the  book  is  admirably  adapted.  Its  value 
is  further  enhanced  by  the  numerous  carefully-drawn  cuts,  which  add  greatly  to  its 
beauty." 

From  the  Boston  Globe. 

"  The  volume  seems  well  adapted  to  the  needs  of  students  who  like  to  have  their 
knowledge  vitalized  by  experiment.  The  fact  that  nearly  all  the  experiments  described 
are  new,  and  have  been  tested,  is  an  additional  recommendation  of  this  handy  volume. 
The  illustrations  add  to  its  interest  and  value,  and  its  simplicity,  both  of  design  and 
execution,  will  commend  it  to  beginners  and  others  seeking  information  on  the  subject." 

From  the  'Philadelphia  Press. 

"It  supplies  a  large  number  of  simple  and  entertaining  experiments  on  the  phe- 
nomena of  light,  that  any  one  can  perform  with  materials  that  maybe  found  in  any 
dwelling-house,  or  that  may  be  bought  for  a  small  sum  in  any  town  or  city.  This 
actually  is  philosophy  in  sport,  which  thoughtful  or  ready  minds  can  easily  convert 
into  science  in  earnest." 


D.  APPLETON  &  CO.,  549  &  551  BROADWAY,  NEW  YORK. 


THE  EXPERIMENTAL  SCIENCE  SERIES. 


SOUND: 


A  Series  of  Simple,  Entertaining,  and  Inexpensive  Experiments  in  the 
Phenomena  of  Sound,  for  the  Use  of  Students  of  Every  Age. 

By  ALFRED   MARSHALL   MAYER, 

Professor  of  Physics  in  the  Stevens  Institute  of  Technology;  Member  of  the 
National  Academy  of  Sciences,  etc. 

UNIFORM  WITH  "LIGHT,"  FIRST  VOLUME  OB1  THE  SERIES. 

Neat  12mo  volume,  bound  in  cloth,  fully  illustrated.    Price,  $1.00. 


"  It  would  really  be  difficult  to  exaggerate  the  merit,  in  the  sense  of  consum- 
mate adaptation  to  its  modest  end,  of  the  little  treatise  on  '  Sound '  which  forms 
the  second  number  of  Appletons'  'Experimental  Science  Series.1  The  purpose 
of  these  hand-books  is  to  teach  the  youthful  student  how  to  make  experiments 
for  himself,  without  the  help  of  a  trained  operator,  and  at  very  little  expense. 
How  successful  the  authors  were  in  attaining  that  end  is  attested  by  the  remark- 
able and  constantly-increasing  demand  for  the  initial  volume.  These  hand-books 
of  Professor  Mayer  should  be  in  the  hands  of  every  teacher  of  the  young."— New 
York  Sun. 

"  The  present  work  is  an  admirably  clear  and  interesting  collection  of  experi- 
ments, described  with  just  the  right  amount  of  abstract  information  and  no  more, 
and  placed  in  progressive  order.  The  recent  inventions  of  the  phonograph  and 
microphone  lend  an  extraordinary  interest  to  this  whole  field  of  experiment, 
which  makes  Professor  Mayer's  manual  especially  opportune."— Boston  Courier. 

"Dr.  Mayer  has  written  a  second  beautiful  book  of  experimental  science,  the 
subject  being  *  Sound.'  It  is  a  little  volume,  is  surprisingly  comprehensive,  and, 
although  intended  for  beginners,  contains  many  pages  that  will  be  read  with 
pleasure  by  those  most  familiar  with  the  subject."— N.  Y.  Independent. 

"  l  Sound '  is  the  second  volume  of  the  '  Experimental  Science  Series.'  Like 
its  predecessor,  it  is  deserving  of  hearty  commendation  for  the  number  of  inge- 
nious and  novel  experiments  by  which  the  scientific  principles  are  illustrated. 
These  little  volumes  are  the  best  manuals  ever  written  for  the  use  of  non-scien- 
tific students,  and  their  study  will  more  than  repay  the  labor  devoted  to  them." 
—Boston  Gazette. 

"  An  interesting  little  treatise  on  '  Sound.'  A  carefully-prepared  price-list  of 
articles  needed  for  tests  and  experiments  adds  to  the  value  of  the  volume."— 
Boston  Evening  Transcript. 


D.  APPLETON  &  CO.,  PUBLISHERS,  649  &  551  BROADWAY,  K.  Y. 


INTERNATIONAL  SCIENTIFIC  SERIES. 


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